JERUSALEM -- Israeli warplanes carried out a strike deep inside Syrian territory on Wednesday, American officials reported, saying they believed the target was a convoy carrying sophisticated antiaircraft weaponry on the outskirts of Damascus that was intended for the Hezbollah Shiite militia in Lebanon.
The officials, who spoke on condition of anonymity, said the Israelis had notified the Americans about the attack, which the Syrian government called an act of "Israeli arrogance and aggression" that raised the risks that the two-year-old civil conflict in Syria could spread beyond the country's borders.
In a statement, the Syrian military said a scientific research facility in the Damascus suburbs had been hit and denied that a convoy had been the target.
Israeli officials declined to comment on the airstrike. But they have been warning that they are monitoring the possible movement of weapons in the Syrian conflict, including chemical weapons, and would take action to thwart any possible transfers into Hezbollah's possession.
It was the first time in more than five years that Israel's air force had attacked a target in Syria, which has remained in a technical state of war with Israel although both sides have maintained an uneasy peace along their decades-old armistice line.
Hezbollah, which plays a decisive role in Lebanese politics, has long relied on Syria as both a source of weapons and a conduit for weapons flowing from Iran. Hezbollah has supported the Syrian government of President Bashar al-Assad throughout the uprising against him in part because it does not want to lose that weapons corridor, and some analysts say that Hezbollah may be trying to stock up on weapons now in case Mr. Assad falls. Other analysts say that Hezbollah would be cautious now about receiving arms from Syria because it does not want to risk drawing an Israeli attack or destabilizing its political position in Lebanon.
Hezbollah's leader, Hassan Nasrallah, recently urged Lebanese citizens to welcome Syrian refugees regardless of their political affiliation, a move widely interpreted as aimed in part at preserving its relationship with Syria in the event of a rebel takeover, in addition to maintaining political calm in Lebanon.
Hezbollah is believed to have replenished and increased its weapons stocks after the 2006 war with Israel, in which Israeli bombardments destroyed some of its arms and other missiles were used to unleash a barrage that killed Israelis as far south as Haifa and drove residents of northern Israel into shelters.
The Syrian statement, carried by state television, said an unidentified number of Israeli jets flying below radar had hit the research facility, killing two people and causing "huge material damage."
"Israeli warplanes violated our airspace at dawn, bombing directly one of the research scientific centers in the Jimraya district in rural Damascus," the Syrian statement said, calling it a "breach of Syrian sovereignty."
It cast the attack as "another addition to the history of Israeli occupation, aggression and criminality against Arabs and Muslims."
"The Syrian government points out to the international community that this Israeli arrogance and aggression is dangerous for Syrian sovereignty and stresses that such criminal acts will not weaken Syria's role nor will discourage Syrians from continuing to support resistance movements and just Arab causes, particularly the Palestinian issue," the statement said.
Israelis have expressed increasing concern in recent days about what they called the threat of chemical or advanced conventional weapons leaking from Syria to Hezbollah in Lebanon or into the hands of extremist Islamic rebel groups as a result of the turmoil in Syria.
The Lebanese Army said in a statement on Wednesday that Israeli warplanes had carried out two sorties, circling over Lebanon for hours on Tuesday and before dawn on Wednesday, but made no mention of any attacks.
Jerusalem has long maintained a policy of silence on pre-emptive military strikes. It would not comment after Sudan accused the Israel military of carrying out an air attack that destroyed a weapons factory in Khartoum, the Sudanese capital, in October. Israel also never admitted to the bombing of a Syrian nuclear reactor in 2007, and Syria kept mum about that attack. The ambiguity allowed that event to pass without Syria feeling pressure to retaliate.
The heightened sense of alert in Israel this week had focused on the Syrian government's precarious hold on its stockpiles of chemical weapons. But Israeli officials and experts have also voiced worry about the fate of what they describe as conventional "strategic weapons" in Syria, including advanced ground-to-air missiles, shore-to-sea missiles and anti-tank missiles. They say such weapons in the hands of Hezbollah could upset the current balance of forces in the region.
Amnon Sofrin, a retired brigadier general and former Israeli intelligence officer, told reporters in Jerusalem on Wednesday that Hezbollah, which is known to have been storing some of its more advanced weapons in Syria, was now eager to move everything it could to Lebanon. He said Israel was carefully watching for convoys transferring weapons systems from Syria to Lebanon.
Israel's air force chief, Maj. Gen. Amir Eshel, said on Tuesday that Syria was a prime example of "the weakening governance in neighboring countries that heralds greater exposure to hostile activity."
Speaking at an international space conference in Israel, General Eshel said: "We work every day in order to lessen the immediate threats, to create better conditions so that we will be victorious in future wars. This is a struggle in which the Air Force is a central player, from here to thousands of kilometers away."
There have been reports in the last week of feverish security consultations between Israel's political and security chiefs, and at least one Iron Dome anti-rocket missile defense battery was deployed in northern Israel. Israel's national security adviser, Yaakov Amidror, was in Moscow for talks with Russian officials on Monday.
Israel has made it clear that if the Syrian government loses control over its chemical weapons or transfers them to Hezbollah, Israel will most likely be compelled to act. Avi Dichter, the minister for the home front, told Israel Radio on Tuesday that options to prevent Syria from using or transferring the weapons included deterrence and "attempts to hit the stockpiles."
Isabel Kershner reported from Jerusalem, Michael R. Gordon from Washington and Rick Gladstone from New York. Reporting was contributed by Anne Barnard, Hania Mourtada and Hwaida Saad in Beirut, and Eric Schmitt in Washington.
This article originally appeared in The New York Times.