BEIRUT -- In what the opposition called the worst atrocity of Syria's civil war, antigovernment activists accused the government of killing hundreds of civilians, including many women and children, in a poison-gas attack targeting pro-rebel Damascus suburbs.
The Syrian government called reports of a massacre untrue, but the scale of the alleged carnage and the graphic videos of the dead and injured that surfaced Wednesday left many officials across the globe demanding action.
If verified, such a massive gas attack could alter the international response to the war that has raged since March 2011. Last year, President Obama called the potential use of chemical weapons in Syria a "red line" that could prompt U.S. intervention.
The U.S. has provided humanitarian and nonlethal aid to the rebels but has been reluctant to get more deeply involved. Despite a declaration in June that it would start providing military assistance, rebels say they have yet to receive any such aid, nor have they been told what to expect or when they will get it.
The opposition said rockets tipped with some kind of apparent nerve agent rained down overnight on areas to the east and south of the Syrian capital, all strongholds of rebels fighting to overthrow the government of President Bashar Assad.
Video uploaded onto YouTube showed rows of bodies, some arrayed on the floors of makeshift clinics. Many were children in underwear and pajamas, purported victims of a barrage that allegedly occurred about 3 a.m. Other footage showed people choking, flailing their arms uncontrollably, rolling their eyes, foaming at the mouth and exhibiting other signs of what could be the effects of chemical poisoning. Most showed no indication of wounds or bleeding.
In one clip, a distraught man cradled what was described as the corpse of his daughter, asking why it had happened.
Each side in the conflict has accused the other of using chemical weapons, and both deny it. The U.S. and its allies have said that evidence indicates the Syrian military has used small amounts of sarin, a nerve agent, on several occasions.
Experts who had been skeptical of previous claims said the new images showed more convincing signs of a chemical attack. But they raised a number of questions that could not immediately be answered. Some suggested the pictures suggested use of a low-grade agent.
The United States and other nations urged that a United Nations team that arrived in Damascus over the weekend to investigate previous charges of chemical weapons use be ordered to look into this incident as well.
The U.N. Security Council convened a two-hour emergency session. But afterward, the U.N., which long has been deeply divided on Syria, issued a statement condemning any use of chemical weapons as a "violation of international law," calling for a "thorough and prompt investigation" and renewing calls for a cease-fire in Syria.
The major U.S.-backed Syrian opposition group said more than 1,300 people were killed, while other antigovernment groups put the number in the hundreds. Such numbers would seem to represent the largest single-day death toll in a conflict that the U.N. says has already cost more than 100,000 lives.
One activist who lives near the town of Arbeen said rockets began hitting the area about 2:30 a.m. Residents who had been sleeping in their basements to protect themselves from shelling could not escape the chemicals, he said.
Another, reached by Skype in Zamalka, said he was awake when the first rocket hit his area. He and his friends, who are part of a volunteer ambulance team, rushed to the scene. Families stumbled out of their homes and into the street, still dressed in their pajamas, choking and out of breath, he said.
By early morning, drugs at three makeshift hospitals had run out, and victims were being treated with water, he said, adding that medical personnel were attempting to wash out people's eyes and mouths with soda.
"These reports are uncorroborated and we are urgently seeking more information," British Foreign Minister William Hague said in a statement in London. "But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria."
The official Syrian news agency called the reports untrue and designed to derail the ongoing U.N. inquiry.
A Syrian military official appeared on state television denouncing the reports as a desperate opposition attempt to make up for rebel defeats on the ground. After months in which rebels were making steady gains against the government, pro-Assad forces have held the momentum for much of this year, regaining territory around Damascus and in several other parts of the country.
Russia, a close ally of Assad, labeled the accusation a "premeditated provocation." Moscow has backed the Syrian government's contention that it is antigovernment rebels, not the military, who have previously used toxic gas. A Russian investigation indicated that rebels had produced chemical arms using a "cottage industry" approach, Moscow said.
Any expanded U.N. inquiry would require approval of the Syrian government. Some kind of safe passage would have to be arranged for U.N. inspectors to enter what are heavily contested war zones. The U.S. and other governments called on Damascus to agree to a new inquiry.
"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 the site," said Josh Earnest, a White House spokesman.
Jean Pascal Zanders, a Belgian chemical weapons expert formerly with the European Union Institute for Security Studies, said the victims' pinkish-blue faces, unfocused eyes and signs of involuntary urination were indications of poisoning by asphyxiation.
"It is clear that something terrible has happened," Zanders wrote in an online post. "The scenes could not have been stage-managed."
But the experts raised some red flags, including signs that medical staff treating the victims didn't appear to be experiencing any symptoms, which would be expected if a pure nerve agent had been released.
Instead, many believe, the footage seemed consistent with use of an agent less potent than what Western officials believe the Syrian regime has in its stockpiles. The experts suggested that a nerve agent such as sarin could have been mixed with other chemicals, rendering it less toxic but also harder to trace.
"If you water them down and add some other chemicals, it makes detection more difficult," said Gwyn Winfield, editor of CBRNe World magazine, which focuses on chemical weapons issues.
Several experts suggested that the timing of the alleged attack -- just three days after the arrival of the U.N. investigators -- further muddied the picture. It was unclear what Assad would gain from such an action, given that a deliberate chemical strike would enrage both his many international enemies and even his allies in Moscow.
Some members of Congress renewed calls Wednesday for the Obama administration to intervene more decisively in the Syrian conflict.
Democratic Sen. Bob Casey of Pennsylvania, co-chairman of a caucus on weapons of mass destruction, said in a statement that Obama's red line appeared to have been crossed. "I continue to believe that the U.S. should consider assertive ways to end the atrocities committed against the Syrian people," he said.
But Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, this week offered a pessimistic assessment of U.S. military options. He argued that while U.S. forces could easily defeat Assad's air defenses and tilt the conflict in favor of the rebels, the United States should avoid even limited military engagement because the rebels, who include fighters loyal to Al Qaeda, don't back U.S. interests.
"Syria today is not about choosing between two sides but rather about choosing one among many sides," Dempsey wrote in a letter to Rep. Eliot L. Engel (D-N.Y.) that was released Wednesday.
Among the areas allegedly targeted Wednesday, according to the opposition, were the suburbs of Jobar, Zamalka and Ain Tarma in the so-called eastern Ghouta region, and Muadhamiya in the southwest. All of the areas are known as antigovernment strongholds.
Syrian authorities have acknowledged carrying out a major offensive in some of the areas allegedly hit by poison gas. But the government said that only conventional weapons were used. For months, the Syrian military has been engaged in a methodical offensive aimed at pushing insurgents away from Damascus and securing the capital from rebel mortar strikes and other attacks.
Times staff writer Raja Abdulrahim in Cairo and special correspondent Alexandra Sandels in Stockholm contributed to this report.