Syrian anti-government activists accused the regime of carrying out a toxic gas attack Wednesday that killed at least 100 people, including many children as they slept, during artillery and rocket barrages on Damascus suburbs.

The attack coincided with the visit by a 20-member U.N. chemical weapons team to Syria to investigate three sites where attacks allegedly occurred during the past year. Their presence raises questions about why the regime — which called the claims of the attack Wednesday “absolutely baseless” — would use chemical agents at this time.

Shocking images emerged from the purported attack, showing lifeless bodies of children lined up on floors of makeshift hospitals and others with oxygen masks on their faces as they were attended to by paramedics. One appeared to be a toddler clad in diapers. There was no visible blood or wounds on their skin.

If confirmed, it would be the deadliest chemical attack in Syria’s civil war. There were conflicting reports, however, as to what exactly transpired and the death toll ranged from a hundred to 1,300. Syria’s Information Minister called the activists’ claim a “disillusioned and fabricated one whose objective is to deviate and mislead” the U.N. mission.

France’s president demanded the United Nations be granted access to the site of Wednesday’s alleged attack, while Britain’s foreign secretary said that if the claims are verified it would mark “a shocking escalation of the use of chemical weapons in Syria.”

The White House said the U.S. was “deeply concerned” by the reports. Spokesman Josh Earnest said the White House had requested that the U.N. “urgently investigate this new allegation.”

Syria’s ally Russia, however, described the reports as “alarmist.” Foreign Ministry spokesman Alexander Lukashevich issued a statement in which he said the “aggressive information campaign” laying full blame on the Syrian government suggests that this is a planned provocation aimed at undermining efforts to convene peace talks between the two sides.

The heavy shelling starting around 3 a.m. local time pounded the capital’s eastern suburbs of Zamalka, Arbeen and Ein Tarma, according to the Britain-based Syrian Observatory for Human Rights monitoring group. The bombardment, as well as the sound of fighter jets, could be heard by residents of the Syrian capital throughout the night and early Wednesday. Gray smoke hung over towns in the eastern suburbs.

Observatory director Rami Abdul-Rahman cited activists in the area who said “poisonous gas” was fired in rockets as well as from the air in the attack. He said he has documented at least 100 deaths, but said it was not clear whether the victims died from shelling or toxic gas.

However, he and other opposition groups described a ferocious offensive on the eastern suburbs known as eastern Ghouta, saying “hundreds” of shells and rockets were unleashed by regime forces Wednesday on the region.

Another group, the Local Coordination Committees, said hundreds of people were killed or injured in the shelling. The Syrian National Coalition, Syria’s main opposition group in exile, put the number at 1,300. The group said it was basing its claim on accounts and photographs by activists on the ground.

Such different figures from activists groups are common in the immediate aftermaths of attacks in Syria, where the government restricts foreign and domestic reporting.

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 killed off 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. The government refuses to confirm or deny it possesses such weapons.

In June, the U.S. said it had conclusive evidence that Assad’s regime used chemical weapons against opposition forces. That crossed what President Barack Obama called a “red line,” prompting a U.S. decision to begin arming rebel groups, although that has not happened yet.

The Syrian government denied the claims of a chemical weapons attack Wednesday.

“All what has been said is ridiculous and naive, unscientific, illogical and subjective,” said Information Minister Omran al-Zoubi, speaking to Syrian state television.

He said the organized media campaign was a result of the regime’s successful operations against rebels on the ground.