Activists and the opposition leadership gave widely varying death tolls, ranging from as low as 136 to as high as 1,300. But even the most conservative tally would make it the deadliest alleged chemical attack in Syria's civil war.
For months now, the rebels, along with the United States, Britain and France, have accused the Syrian government of using chemical weapons in its campaign to try to snuff out the rebellion against President Bashar Assad that began in March 2011. The regime and its ally, Russia, have denied the allegations, pinning the blame on the rebels.
The murky nature of the purported attacks, and the difficulty of gaining access to the sites amid the carnage of Syria's war, has made it impossible to verify the claims. After months of negotiations, a U.N. team finally arrived in Damascus on Sunday to begin its investigation into the alleged use of chemical weapons in Syria. But the probe is limited to three sites and only seeks to determine whether chemical agents were used, not who unleashed them.
The timing of Wednesday's attack — four days after the U.N. team's arrival — raised questions about why the regime would use chemical agents now.
The White House said the U.S. was "deeply concerned" by the reports, and spokesman Josh Earnest said the Obama administration had requested that the U.N. "urgently investigate this new allegation."
"If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the U.N. team's immediate and unfettered access to this site," Earnest said.
Almost exactly one year ago, President Barack Obama called chemical weapons a "red line" for potential military action, and in June, the U.S. said it had conclusive evidence that Assad's regime had used chemical weapons against opposition forces.
But the possibility of intervention seemed ever smaller after Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said in a letter this week that the administration is opposed to even limited action because it believes rebels fighting the Assad government wouldn't support American interests.
Russia decried Wednesday's reports as "alarmist." Foreign Ministry spokesman Alexander Lukashevich denounced an "aggressive information campaign" laying full blame on the Syrian government as a provocation aimed at undermining efforts to convene peace talks between the two sides.
The regime began shelling the capital's eastern suburbs of Zamalka, Arbeen and Ein Tarma around 3 a.m. as part of a fierce government offensive in the area, which has a strong rebel presence, according to the Britain-based Syrian Observatory for Human Rights monitoring group.
The heavy thud of artillery and rockets, as well as the grinding roar of fighter jets, could be heard by Damascus residents throughout the night and early Wednesday, and a pall of gray smoke hung over the towns.
Observatory director Rami Abdul-Rahman cited activists in the area who said "poisonous gas" was fired in rockets as well as from the air. He said that he had documented at least 136 deaths, but said it was not clear whether the victims died from shelling or toxic gas.
The Local Coordination Committees activist group said hundreds of people were killed or wounded. The Syrian National Coalition, the main Western-backed opposition group in exile, put the number at 1,300, basing its claim on accounts and photographs by activists on the ground.
George Sabra, a senior member of the Coalition, blamed the regime, as well as "the weakness of the U.N. and American hesitation" for the deaths. "The silence of our friends is killing us," he said, adding that Wednesday's attack effectively eliminated any chance for peace negotiations with the regime.
Syria is said to have one of the world's largest stockpiles of chemical weapons, including mustard gas and the nerve agent sarin.