BEIRUT, Lebanon -- The Syrian government accused rebels Tuesday of killing dozens of civilians in a chemical attack near the northern city of Aleppo, the first allegation of such a devastating chemical strike during the more than two-year conflict.
The opposition vehemently denied the claim and charged that the government of President Bashar Assad was behind the attack.
The alleged chemical assault injects an explosive new issue into the international debate about how to deal with the escalating violence in Syria, where tens of thousands have already died and aid experts have warned of a looming humanitarian catastrophe. Diplomatic efforts to craft a ceasefire have failed; more than 1 million Syrians have fled the country.
President Barack Obama last year warned the Syrian government that any use of chemical weapons -- or a transfer of stockpiles to other powers -- was a "red line" that could trigger a U.S. response. On Tuesday, however, U.S. officials reacted cautiously to the cross-allegations, saying there was no definitive evidence that a chemical attack had been unleashed. "We are looking carefully at the information as it comes in," White House spokesman Jay Carney told reporters.
At the same time, Washington flatly rejected the possibility of a rebel-launched chemical attack and asserted that the Syrian government was capable of conducting such a strike.
Each side in the conflict has routinely blamed the other for massacres and attacks that caused large-scale civilian casualties.
Syria's state-run news agency said al-Qaida-linked "terrorists" -- its standard term for the armed opposition -- had fired a chemically armed rocket early Tuesday that killed 25 and injured 110. The missile, launched from a rebel-controlled district, fell about 300 yards from its apparent intended target, a military post outside Aleppo, the government said.
The explosion unleashed a gas that caused those who inhaled it to have convulsions and lose consciousness, the government said. The state media showed images of patients said to be suffering from irregular breathing, neurological disorders and other symptoms consistent with a chemical attack.
The government seemed to link the strike to a previous rebel seizure of a factory in Aleppo that contained tons of chlorine, a chemical employed in water purification and used for many industrial purposes but also deployed as a toxic gas in World War I. Experts say chlorine gas is not an especially effective weapon because it disperses quickly in the air.
The Reuters news agency reported that apparent victims at a hospital in Aleppo were suffering from breathing problems. The victims said they smelled chlorine after the attack.
Opposition spokesmen labeled the attack a government scheme to discredit the opposition and pave the way for even larger chemical bombardment. An aide to Gen. Salim Idriss, who heads a rebel military council, said insurgents lacked the know-how and material to launch a chemical assault. He blamed the Syrian regime and said Tuesday's attack was probably a precursor to more widespread government deployment of chemical weapons.
"The regime tries to do something on a small scale, and if the international community doesn't condemn it, they may use it on a much wider scale," said the aide, reached by phone in Turkey, who declined to be named for security reasons.
U.S. authorities seemed to side with the rebel version but did not accuse the government directly. The State Department said it had no indication that the Syrian rebels possessed chemical weapons.
The Syrian government is widely believed to hold substantial chemical stockpiles, although Damascus has never directly acknowledged having a chemical armory. U.S. officials did not rule out the possibility that the government may have launched a chemical strike and tried to blame it on the opposition.
"We've been concerned that the regime, in failing to achieve its military objectives even by the most barbaric conventional means, may get desperate enough to use these weapons," U.S. State Department spokeswoman Victoria Nuland said in Washington.
In Moscow, Russian officials endorsed Damascus' version that rebels had launched a chemically loaded missile outside Aleppo. The Russian Foreign Ministry labeled the episode an "extremely alarming and dangerous development."
Washington has backed the ouster of Mr. Assad; Moscow has resisted opposition efforts to topple its longtime ally in Damascus.
Meanwhile, the major Syrian opposition group Tuesday elected Ghassan Hitto, a U.S.-educated Syrian activist, to serve as prime minister of rebel-controlled areas of Syria.
Mr. Hitto, a native of Damascus, has been living in the United States for decades, working recently as a technology executive. He is little known outside Syrian exile circles, but he has been active in humanitarian efforts to aid Syrians during the nation's two-year conflict.
Mr. Hitto was named to the post in an election at a meeting in Istanbul of the U.S-backed umbrella group known as the National Coalition for Syrian Revolutionary and Opposition Forces. The group was formed last year amid pressure from the United States and other allies to unify the often-fractious ranks of the Syrian opposition.
Some opposition factions remain outside of the coalition and do not recognize its authority. Syrian exiles dominate the coalition.