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ITN

Hundreds of people are believed to have been killed in an apparent gas attack on rebel-held parts of eastern Damascus that is thought to be the most significant use of chemical weapons since thousands of Kurds were gassed by Saddam Hussein in Halabja 25 years ago.

Medics, as well as opposition fighters and political leaders, said the death toll had reached 1,400 and was likely to rise further with hundreds more critically wounded in districts besieged by the Syrian military. Other estimates put the current death toll at between 200 and 500. None of the figures could be independently verified. On Thursday morning rebels said new bombardments of rockets and mortars struck neighbourhoods hit by the gas attack.

The Syrian government acknowledged it had launched a major offensive in rebel-held districts in the east of the capital – described by pro-regime media as the biggest since the start of the civil war – but strongly denied using chemical weapons.

"These are lies that serve the propaganda of the terrorists," a Syrian official said, referring to the armed opposition. "We would not use such weapons."

However, George Sabra, the head of the main Syrian opposition group, laid the blame squarely at the Assad regime, saying the scenes "constitute a turning point in the regime's operations".

"This time it was for annihilation, rather than terror," he said.

Location of Wednesday's attack. Credit: Guardian graphics

International reaction intensified throughout the day. The UN security council called an emergency session and the White House formally requested the UN to investigate the attack. William Hague, the foreign secretary, said the UK was "deeply concerned".

The UN secretary-general Ban Ki-moon called for "a thorough, impartial and prompt investigation" of allegations of chemical weapons use.

UN deputy spokesman Eduardo del Buey said earlier that the secretary-general was "shocked" at the alleged use of chemical weapons and is determined to ensure a "thorough investigation" of all reported incidents.

After a two-hour, closed-door meeting, the council president said there was "strong concern" about the allegations "and a general sense that there must be clarity on what happened."

A UN inspection team arrived in Damascus this week to look into earlier claims of chemical weapon use, but was granted permission to enter Syria with a limited mission to investigate only three specific sites. An expanded mandate to investigate Wednesday's attack in eastern Ghouta – only 10 miles from the team's hotel – must be sought by the UN secretary general and then approved by Syria.

The US moved quickly to make the request. The White House said: "For the UN's efforts to be credible they must have immediate access to witnesses and affected individuals, and have the ability to examine and collect physical evidence without any interference or manipulation from the Syrian government. If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the UN team's immediate and unfettered access to this site."

Rescuers and victims said the shelling of eastern Ghouta started shortly after 2am and targeted three districts, Ein Tarma, Zermalka and Jobar, all rebel strongholds for the past year.

"It was around 2.30am Wednesday when we received calls from Zemalka and Jobar," said a Free Syria Army (FSA) officer, Captain Alla'a al-Basha, who has documented previous alleged chemical attacks in the area.

"The FSA members were asking for more forces to evacuate the civilians as the shells were coming in at around five per minute. As soon as I and my team arrived at the scene, I saw bodies scattered in the streets. I saw whole houses – none of their residents were alive. When I got there, I could smell what seemed to be burning sulphur and something like cooked eggs. The smoke was not pure white.

"Most of the victims were shivering and they turned yellow. I saw a woman who was tearing at her clothes as she could not breathe. The number of the casualties that we were able to document so far is 1,228 martyrs. The doctors think that more than 20 shells were fired with fatal gases.

"Most of the victims did not appear to be injured but died out of suffocation. I held a young boy whose body was like a piece of wood and his colour was very blue. He did not have any wound."

By Wednesday night, more than 120 videos had been uploaded to the internet, most depicting scenes of men women and children in respiratory distress, on watery floors, and doctors describing the victims' symptoms. Other videos showed scores of bodies wrapped in white shrouds, or lying on grey concrete. White foam was bubbling from the mouth and nostrils of many victims. Some writhed in distress, apparently struggling to breathe.

Doctors at makeshift clinics said they were working without oxygen and had been overrun by the number of victims, many of whom needed lifesaving treatment that they could not provide.

Treatment of victims appeared rudimentary, with water and vinegar among the means of trying to dilute the effects. "We know when we have an area targeted by fatal gases we would take plastic masks and put wet cloths on our noses and mouths," said Basha. "But most of the civilians do not know that they have to do that."

Sergeant Abu Ali, who runs a field hospital in the Nashabiya area of eastern Damascus, said he had received patients who were vomiting and had high temperatures, breathing problems, limb stiffness and were in comas. "We received 60 cases. Most of them were sent to the nearby farms after their situation was stabilised and those with acute symptoms were kept here. I have very few medicines and all the oxygen tubes I have had run out now. People need intensive care."

One witness told Reuters: "We would go into a house and everything was in its place, every person was in their place. They were lying where they had been. They looked like they were asleep. But they were dead."

Ralf Trapp, a consultant on chemical and biological weapons, said getting access to the scenes of the attacks was paramount for inspectors. "The logical thing to do would be to go in and start interviewing doctors and getting blood and urine samples.

"This is the ideal moment to collect samples because it is so shortly after the attack. They may get intact agent – in the first day or so you would still find intact sarin, for example.

"Within a few days, you would find degradation products. If you link those to clinical examinations and testimony, you can build up a very precise picture of what happened.

"They need to try to get to the site where it happened, talk to people who were on the spot when it happened, to victims and observers, to create as complete a picture of the actual attack. They want to discriminate against other types of weapons that might cause similar effects or release something by chance."

Charles Duelfer, a former US chief weapons inspector, said: "[Video] reports of doctors treating these people, that's real data." Duelfer said the scale of the attack could probably be proved by the intelligence community. "It will be pretty clear pretty quickly because various countries' intelligence apparatus will have noticed something on this scale, whether it's artillery, rockets, or shells. These are knowable things."The White House is going to be hard pressed to construct an answer to this one. It was easy to waffle a bit so long as alleged use was minor and didn't happen again, but this is really putting the administration in a corner.""