BEIRUT, Lebanon -- Four explosions just west of Damascus shook the ground across the Syrian capital early on Sunday, sending fiery mushroom-shaped clouds towering over the landmark Mount Qasioun and brightening the night sky in a demonstration of firepower more potent than anything the residents of the city, a government stronghold, have witnessed during more than two years of war.
The Syrian government immediately blamed Israel for the explosions, whose power appeared to far outstrip that of any weapons in the rebels' arsenals. Israeli officials refused to confirm that Israeli forces had carried out the strikes, which the Syrian deputy foreign minister, speaking on CNN, called "an act of war."
With much still unexplained about the effects and motivations of the attack, it rattled the region, which has lived in fear that the Syrian war will lead to a wider conflict. It was unclear whether Israel, if it carried out the strikes, sought to intervene in Syria's civil war or was simply expanding its campaign to prevent the Syrian government from transferring weapons to Hezbollah, the militant Shiite organization in Lebanon that is Syria's ally and one of Israel's most dangerous enemies.
The attacks could end up providing psychological and perhaps military assistance to the Syrian rebels, who over the last several weeks have faced losses in a series of government offensives around Damascus and the city of Homs to the north. For the rebels, any damage to crucial military structures from the attacks -- said by opposition activists to have hit bases of elite troops as well as weapons stores -- would be offset by political complications if the explosions are linked to Israel.
Israel deployed two of its Iron Dome missile-defense batteries in northern cities, and Prime Minister Benjamin Netanyahu avoided any mention of the developments in Syria in his remarks at the start of the weekly cabinet meeting on Sunday. But he delayed his scheduled departure to China for two hours so he could convene his security cabinet.
The attack would be the third Israeli strike in Syria this year and the second in two days. An American official said a more limited strike early Friday at Damascus International Airport was meant to destroy weapons being sent from Iran to Hezbollah. That was also the goal of a similar Israeli strike in January.
But the explosions that struck Damascus on Sunday, shaking the ground across the city, appeared to be of far greater magnitude and potentially broader political and military significance.
The rebels' Damascus Military Council quickly sought to capitalize on the blasts. The council issued a statement calling on all fighters in the area to work together, put aside rivalries and mount focused attacks on government forces that have so far kept a solid hold on the capital.
Still, military analysts said the attacks by themselves were not likely to tip the balance between the rebels and the Syrian president, Bashar al-Assad. But Louay Mekdad -- a spokesman for the Supreme Military Council who is considered Washington's best option to become a military ally among the rebels -- discounted claims by some rebel groups that they would take advantage of the strikes to escalate their attacks on the government.
Video posted online on Sunday showed multiple explosions in the area west of Damascus, which is home to an array of crucial military installations. According to residents in the area and Syrian opposition groups, the attacks not only struck bases of the army's elite fighting units, but also a research center that American officials have said is the country's main facility for the development of chemical weapons.
Not all of those assertions were confirmed by the Syrian government, which said only that the research center in Jamraya had been hit.
Hassan Husseini, 41, a Damascus resident reached by Skype, said he and his family heard three explosions that began around 2 a.m.
"Luckily we had the windows open at home or they would have been destroyed by the intensity and pressure of it," he said, adding that he felt "the walls moving and the ground trembling beneath us."
"The electricity and the lights went on and off several times for a few seconds, and then we started smelling burnt gunpowder," he said.
Syrian state television said the explosions confirmed government contentions that the rebels are part of an American-Israeli conspiracy to topple Mr. Assad for his support of Palestinians and his opposition to Western policies in the Middle East.
While being seen as allies of Israel could tarnish the rebels in Syrian eyes, the opposition could point to the strikes as proof of their government's hypocrisy. A frequent refrain among fighters and activists has been that although the government's security forces and military failed to prevent the Israeli strikes -- and for that matter have not clashed with Israel since 1973 -- they have killed tens of thousands of Syrians and jailed many more in order to hold onto power.
Some rebels say openly that they consider Mr. Assad a higher priority target than Israel, while making clear that they do not embrace Israel. The main exile Syrian opposition coalition walked that line carefully in a statement issued after the bombings, blaming the government for allowing attacks by "external occupying forces."
"The regime has used its forces to suppress the popular demands of the people for change, weakening Syrian defense, and thus allowing external occupying forces to hit Syrian locations," the statement said. "Israel's actions, including its pre-emptive attacks to weaken Syrian defenses, demonstrate a fear of losing the years of peace that the Assad regime provided for Israel."
The conflict has taken on an increasingly sectarian cast, and some opposition fighters have said that for the Sunni-led rebellion, the greatest enemy is not Israel but Iran and Hezbollah, which are dominated by Shiites and are the closest allies of Mr. Assad's government. In recent weeks, the Sunni fighters have increased their criticism of Shiites and Alawites, a related sect to which Mr. Assad belongs.
Government supporters have also portrayed the rebellion as driven by extremist Sunni ideologies like Wahhabism. The opposition has accused Alawite and Shiite militias of carrying out massacres of Sunnis, including in the last two weeks in the Damascus suburbs and in Baniyas, a mostly Sunni area in the largely Alawite coastal region.
Iran's defense minister, traveling inside the country with President Mahmoud Ahmadinejad, accused the United States of "giving the green light" for the airstrikes. The minister, General Ahmad Vahidi, told Iran's semiofficial Fars News agency that is becoming "increasingly clear" to Iran that Israel and Syrian rebels are working together to bring down the Assad government.
"These attacks reveal the close relations between the mercenary terrorists and their supporters, the Zionist regime," he said. While Mr. Vahidi did not elaborate, Iranian officials have often accused Syrian rebel forces of providing assistance and intelligence on the ground to pave the way for Israeli airstrikes.
In a sign of Iran's resolve to help the Assad government, the commander of the Iran Army's ground forces, General Ahmad Reza Pourdastan, reiterated statements by Iranian officials that Iran was ready to provide military training to the Syrian Army.
"As a Muslim nation, we back Syria, and if there is need for training we will provide them with the training, but won't have any active involvement in the operations," he said in remarks reported by the state Islamic Republic News Agency.
While Israeli government and military officials refused to confirm or to take any responsibility for the strikes, one senior Israeli official who spoke on the condition of anonymity said that he did not think that Israel was entering a war with Syria and suggested that Syria was unlikely to respond.
President Assad "has his own problems," the official noted. "He doesn't need Israel in the mess."
Similarly, the official said he did not think that Hezbollah would respond. "Hezbollah, as far as understood, has no intention of opening a war in Israel," the official said, adding that it was keeping its Iranian-supplied weapons for any attack by Israel or the United States.
Israeli experts said Israel was only interested in protecting its immediate interests, and that the latest strikes appeared to have more to do with Israel's cardinal standoff against Iran.
"This shouldn't be seen as Israel intervening on behalf of the rebels or against Bashar," said Jonathan Spyer, a senior research fellow at the Interdisciplinary Center in Herzliya, Israel. "This is an escalation in a conflict we know about, and that is the conflict between Israel and Iran, the long shadow war, as people call it. This is an incident in that war."
But, he said, "one has to ask oneself about Israel's calculus."
"Obviously there is a risk in that at a certain point a response becomes more likely," he added.
Professor Moshe Maoz of the Hebrew University said that Iran was now the crucial actor in how the conflict unfolded.
"If Hezbollah gets a green light from Iran to retaliate, Israel won't remain idle, and it could lead to regional war," he said. "The decision is probably to be taken in Tehran."
Israeli analysts said that Mr. Assad would be wary of retaliating against Israel, aware that Israel could strike back with far more firepower than the rebels in Syria might muster. At the same time, they said, Israel was taking a calculated risk because if strikes grow more frequent and stronger, the more likely it is that they will lead to a reaction.
"It is the kind of thing that you know how it begins, but not how it ends," said Professor Eyal Zisser of Tel Aviv University said.
"Israel is still not involved in the war in Syria," he added, "but it is getting closer."
Anne Barnard reported from Beirut, and Jodi Rudoren and Isabel Kershner from Jerusalem. Contributing reporting were Thomas Erdbrink from Tehran, Ben Hubbard from Cairo, Michael Schwirtz from New York, Hania Mourtada and Hwaida Saad in Beirut, Hala Droubi in Dubai and an employee of The New York Times from Damascus.
This article originally appeared in The New York Times.