The images showed lifeless children — wrapped in simple white cloths, their pale faces unmarked by any wound — lined up shoulder to shoulder in a vivid demonstration of what activists say was an attack Wednesday by the Syrian regime that killed at least 130 people with toxic gas.

The Syrian government has adamantly denied using chemical weapons in an artillery barrage targeting suburbs east of Damascus, calling the allegations "absolutely baseless." The United States, Britain and France have demanded that a team of UN experts already in the country be granted immediate access to investigate the claims.

The UN Security Council held emergency consultations about the purported attack, and UN deputy spokesman Eduardo del Buey said the head of a UN team sent to Damascus to investigate earlier claims of chemical attacks was in talks with the Syrian government.

Videos and photographs showed row upon row of bodies wrapped in white shrouds lying on a tile floor, including more than a dozen children. There was little evidence of blood or conventional injuries and most appeared to have suffocated. Survivors of the purported attack, some twitching uncontrollably, lay on gurneys with oxygen masks covering their faces.

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.

Syrian women light candles during a vigil against the alleged chemical weapons attack on the suburbs of Damascus, in front the United Nations headquarters in Beirut, Lebanon. (Hussein Malla/Associated Press)

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 UN 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 UN team's arrival — raised questions about why the regime would use chemical agents now.

U.S. 'deeply concerned' by reports

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 UN "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 UN team's immediate and unfettered access to this site," Earnest said.

Canada concerned Foreign Affairs Minister John Baird said Canada is concerned by reports of a deadly poison gas attack. "Such reports are extremely concerning, and we will continue to monitor the situation closely with our allies and to seek further information," he said. "Such an attack is completely unacceptable, and we call on the Assad regime to co-operate with the UN officials investigating these disturbing reports." Baird said Canada is supporting the UN team in Syria sent to investigate previous reports of the possible use of chemical weapons by providing funds to the Organization for the Prohibition of Chemical Weapons.

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.

Shelling in Damascus suburbs

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 grey smoke hung over the towns.

A man, affected by what activists say is nerve gas, breathes through an oxygen mask in the Damascus suburbs of Jesreen. Syrian activists accused President Bashar al-Assad's forces of launching a gas attack on Wednesday. (Ammar Dar/Reuters)

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 Co-ordination 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 UN 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.

Jean Pascal Zanders, an independent researcher who specializes in chemical and biological weapons and disarmament, said that in videos of the aftermath of the attacks, the hue of the victims' faces appeared to show many suffered from asphyxiation.

However, he said, the symptoms they exhibited were not consistent with mustard gas or the nerve agents VX or sarin. Mustard gas would cause blistering of the skin and discoloration, while the nerve agents would produce severe convulsions in the victims and also affect the paramedics treating them — neither of which was evident from the videos or reports.

"I'm deliberately not using the term chemical weapons here," he said. "There's plenty of other nasty stuff that was used in the past as a chemical warfare agent, so many industrial toxicants could be used too."

A pharmacist in the town of Arbeen who identified himself as Abu Ahmad said he attended to dozens of wounded people in a field hospital after the shelling on Zamalka and Ein Tarma early Wednesday. He said many were moved to Arbeen.

The bodies of 63 of the dead had signs of a chemical weapons attack, he said, though he could not confirm this.

"Their mouths were foaming, their pupils were constricted, and those who were brought in while still alive could not draw their breaths and died subsequently," he told The Associated Press via Skype. "The skin around their eyes and noses was greyish."

Activists in nearby Zamalka told Abu Ahmed that an additional 200 people died in that town on Wednesday.

Syria denies using chemical agents

Syria's information minister, Omran al-Zoubi, denied government troops used chemical agents, calling the activists' claim a "disillusioned and fabricated one whose objective is to deviate and mislead" the UN mission.

The head of the UN team, which has a mandate to investigate previous claims of alleged chemical attacks, said he wants to look into the latest claims. Speaking to Swedish broadcaster SVT, Ake Sellstrom said the high numbers of dead and wounded being reported "sound suspicious."

A survivor from what activists say is a gas attack rests inside a mosque in the Duma neighbourhood of Damascus. (Bassam Khabieh/Reuters)

"It looks like something we need to look into," Sellstrom, who is Swedish, was quoted as saying.

He said a formal request from a member state would have to go through UN channels and Syria would need to agree — and there is no guarantee that it would.

French President François Hollande said the latest allegations "require verification and confirmation," according to government spokeswoman Najat Vallaud-Belkacem. Hollande said he would ask the UN to go to the site "to shed full light" on the allegations.

In addition to the U.S. and Britain, Germany, Turkey and the EU called for immediate UN access to the site of the alleged attack. The Syrian government did not immediately respond to the demands.

Mohammed Saeed, an activist in the area, told the AP via Skype that hundreds of dead and injured people were rushed to six makeshift hospitals in the eastern suburbs of Damascus.

"This is a massacre by chemical weapons," he said. "The visit by the UN team is a joke. ... (Assad) is using the weapons and telling the world that he does not care."

Photos posted on Facebook by an activist group in Arbeen showed rows of Syrian children wrapped in white shrouds, and others with their chests bare. There appeared to be very little sign of blood or physical wounds on the bodies.

In an amateur video posted online, a young girl with curly brown hair wearing a Minnie Mouse shirt lay on the ground, her head lolling on the tile floor as doctors injected medicine into her arm. Next to her, paramedics attended to two young boys who appeared unconscious, their bodies limp.

The photos and videos distributed by activists to support their claims were consistent with AP reporting of shelling in the area, though it was not known whether the victims died from a poisonous gas attack.