The UK, France and the Arab League said they would formally urge UN inspectors currently in Syria to investigate allegations that the Syrian army used chemical weapons on civilians outside Damascus on Wednesday morning.

French President Francois Hollande, speaking at a regular cabinet meeting, said the latest allegations “require verification and confirmation” and that he would ask the UN to go to the site “to shed full light” on the situation, government spokeswoman Najat Vallaud-Belkacem said.

Amid conflicting reports of casualties, the Local Coordination Committees, Syria’s main opposition group in exile, put the number of dead at 1,300. The group said it was basing its claim on accounts and photographs by activists on the ground.

Britain’s Foreign Secretary William Hague said the UK planned to ask the United Nations Security Council to discuss the chemical weapon attack claims. The UN inspectors should be given access to the site, he said, and if the claims were verified, it “would be a shocking escalation of the use of chemical weapons in Syria. We are determined the people responsible will one day be held to account.”

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

However, Swedish chemical weapons expert Ake Sellstrom, who leads the UN team in Syria, said that Syria would need to agree to such an inspection and that a formal request would have to come from a member state and go through UN channels.

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

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.

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.

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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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 20-member UN team 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 was limited: The team would report on whether chemical weapons were used, and if so which ones, but it would 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.