Syrian activists close to the country’s opposition claimed hundreds of people were killed in a devastating “poison gas” attack by regime forces outside Damascus Wednesday.

The attack came as UN chemical weapons inspectors were beginning a probe of chemical weapon use in sites around Syria.

There were several differing reports on the numbers of dead. A Free Syrian Army source told Al Arabiya the death toll stood at 1,188, while the Local Coordination Committees said some 785 people were killed. A nurse at an emergency clinic in Douma told Reuters the death toll was at 213, and the head of the Britain-based Syrian Observatory for Human Rights said 40 were confirmed dead and the death toll could reach over 200.

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Groups quoted activists as saying that regime forces fired “rockets with poisonous gas heads” in the attack.

The Syrian Observatory said the shelling was intense and hit the eastern suburbs of Zamalka, Arbeen and Ein Tarma. Activists told Reuters that Jobar was also targeted. The areas are largely held by rebel forces.

The intensive bombardment as well as the sound of fighter jets could be heard by residents of the Syrian capital throughout the night and early Wednesday, and gray smoke hung over towns in the eastern suburbs.

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Rami Abdul-Rahman, the Syrian 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.

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 use of a chemical agent could not be immediately verified. The government denied it had used chemical weapons, according to a report in the state-run SANA news agency.

An activist group in the town of Arbeen east of Damascus posted on its Facebook page pictures purporting to show rows of Syrian children, wrapped in white death shrouds, and others, with chests bared. There appeared to be very little signs of blood or physical wounds on the bodies.

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

In the hours after the attack dozens of videos were posted to YouTube showing reported victims of the attack, including children. Some videos showed dozens of bodies while others showed doctors and others struggling to treat people having seizures. The veracity of the videos could not be immediately verified.

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British Foreign Secretary William Hague said he was “deeply concerned” by the reports.

The Syrian 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.

The Arab League also urged the UN officials currently in Syria to “immediately” travel to the attack site and conduct an investigation.

The 20-member UN team, led by Swedish chemical weapons expert Ake Sellstrom, arrived in Damascus on Sunday to investigate three sites where chemical weapons attacks allegedly occurred: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations being kept secret for security reasons.

The mandate for the planned probe is limited: The team will report on whether chemical weapons were used, and if so which ones, but it will not determine the responsibility for an attack. This has led some commentators to question the value of the investigation.

Syria is said to have one of the world’s largest stockpiles of chemical weapons, including mustard gas and the nerve agent sarin, though it has never admitted possessing such weapons.

Diplomats and chemical weapons experts have raised doubts about whether the experts will find anything since the alleged incidents took place months ago.

The Syrian government initially asked the UN to investigate an alleged chemical weapons attack on March 19 in Khan al Assal, which was captured by the rebels last month. The government and rebels blame each other for the purported attack which killed at least 30 people.

Britain, France and the US followed with allegations of chemical weapons use in Homs, Damascus and elsewhere. UN Mideast envoy Robert Serry told the Security Council last month that the UN has received 13 reports of alleged chemical weapons use in Syria.