The UN Security Council is holding an emergency meeting after Syrian regime forces fired intense artillery and rocket barrages on the eastern suburbs of the capital of Damascus, in what two pro-opposition groups claimed was a "poisonous gas" attack reported to have killed at least 100 people.
The council was meeting Wednesday afternoon, after Russia and the U.S. made calls for an investigation into the alleged attack.
UN deputy spokesman Eduardo del Buey said Secretary General Ban Ki-moon is "shocked" at the reported use of chemical weapons in the country.
The claims came as a 20-member UN chemical weapons team was in Syria to investigate three sites where chemical weapons attacks had allegedly occurred in the past.
Del Buey said the team's leader, Ake Sellstrom, is in discussions with the Syrian government on all issues related to alleged chemical weapons use, including Wednesday's incident.
The Syrian government has denied the reports of Wednesday's chemical weapons attack as baseless.
"They are an attempt by to divert the UN commission on chemical weapons from carrying out its mission," the state-run SANA news agency said earlier Tuesday, quoting an unnamed government official, as is its standard practice.
The Britain-based Syrian Observatory for Human Rights said the shelling was intense and hit the capital's eastern suburbs of Zamalka, Arbeen and Ein Tarma.
It quoted activists as saying that regime forces fired "rockets with poisonous gas heads" in the attack that killed "tens of people."
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Rami Abdul-Rahman, the Observatory chief, said the activists in the area said "poisonous gas" was fired in rockets as well as from the air. He added that regime forces were on a wide offensive on the eastern and western rebel-held suburbs of Damascus.
It is not clear whether the victims died from shelling or toxic gas attacks, said Abdul-Rahman. But Abdul-Rahman said that the death toll could reach as many as 200 in the suburbs of Damascus.
George Sabra, one of the leading opponents of Assad, said the death toll was 1,300 killed by poison gas rained down on suburbs east of Damascus, Reuters reported.
Regime denies chemical weapons use
The Syrian government has long denied claims by the opposition on chemical weapons use, saying rebels fighting to overthrow President Bashar al-Assad's government have used such weapons.
Following Wednesday's reports, the Observatory called upon the UN team in Syria and all international organizations "to visit the stricken areas and to guarantee that medical and relief supplies reach the people as soon as possible." It also called for an investigation into the attack.
Mohammed Saeed, an activist in the area, told The Associated Press 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," said Saeed. "The visit by the UN team is a joke … Bashar is using the weapons and telling the world that he does not care."
The UN team, led by Swedish expert Ake Sellstrom, is meant to probe three sites: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations, which are being kept secret for security reasons.
Wednesday's claim of the chemical attack, if confirmed, would be the most serious report since the March 19 claim of the attack in Khan al-Assal when at least 30 people were reported killed. Assad's regime and the rebels have blamed each other for that attack.
The unrest in Syria began in March 2011 and later escalated into a civil war. More than 100,000 people have been killed in the conflict so far, according to the United Nations.
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.
With files from Reuters and CBC News