A leading Syrian opposition figure said on Wednesday 1,300 people had been killed in attacks by President Bashar al-Assad's forces around Damascus in which he said chemical weapons had been used.

"Today's crimes are...not the first time the regime has used chemical weapons," George Sabra told a news conference in Istanbul. "But they constitute a significant turning point in the regime's operations...This time it was for annihilation rather than terror."

Syria denied activist reports on Wednesday that the army had used chemical weapons, describing the assertions as completely untrue. Video footage from districts east of the capital showed people choking, some of them foaming at the mouth, and many bodies with no signs of injuries, reported Reuters.

State news agency Sana said "reports on the use of chemical weapons in Ghouta are totally false. It's an attempt to prevent the UN commission of inquiry from carrying out its mission."

The agency described Wednesday's violence as "a series of operations" by army units "against terrorist groups" in Jobar, Irbin and Zamalka, "killing a number of them and destroying their hideouts".

The regime describes its opponents as "terrorists".

Throughout the morning, activists reported chemical attacks in several rebel areas east and southwest of Damascus.

The Local Coordination Committees, a network of activists, reported hundreds of casualties in the "brutal use of toxic gas by the criminal regime".

And in videos posted on YouTube, the Syrian Revolution General Commission, another activist group, showed what it called "a terrible massacre committed by regime forces with toxic gas."

The attack "led to suffocation of the children and overcrowding field hospitals with hundreds of casualties amid extreme shortage of medical supplies to rescue the victims, particularly Atropine," the LCC said.

Eastern Ghouta "was also shelled by warplanes following the chemical attack that is still ongoing, which led to hundreds of casualties and victims, among them entire families," it said.

The Coalition called for an urgent UN Security Council meeting.

"I call on the Security Council to convene urgently," Coalition leader Ahmad Jarba told Al-Arabiya television, condemning what he called a massacre.

British Foreign Secretary William Hague said his country will refer the charges of a chemical weapons strike to the Security Council.

He was "deeply concerned" by the reports and said if they were proved they would mark a "shocking escalation" in the 29-month civil war.

The heavy bombing could be heard in the capital itself, where a grey cloud capped the sky.

The Arab League urged the inspectors to visit the site immediately "to see the reality of the situation and investigate the circumstances of this crime."

UN chief Ban Ki-moon insisted Monday that the inspectors be granted unrestricted access to Syrian sites.

"In order to credibly establish the facts, the mission must have full access to the sites of the alleged incidents," the secretary general said.

Al-Watan newspaper said the government had "pledged to cooperate and facilitate the work" the UN inspectors, who launched their mission Tuesday.

There have frequent claims by anti-regime activists of the alleged use of chemical weapons by the army, particularly in Damascus province and Homs in central Syria.

Claims have also emerged from Idlib in the northwest, while the army and rebels have exchanged blame over the alleged use of chemical arms in a March attack in Khan al-Assal in the northern province of Aleppo.