The UN Security Council is to hold an emergency meeting after claims by Syrian opposition groups that hundreds of people were killed in chemical weapons attacks by Bashar al-Assad's forces on the outskirts of Damascus on Wednesday morning.

The death toll is unclear, with different groups issuing different estimates, but most put the figure in the hundreds. Leading opposition figure George Sabra claimed 1,300 have been killed, while the local co-ordination committees put the death toll at 1,360. Other figures were significantly lower. The Guardian cannot independently verify the figures.

If confirmed, the attack could force Barack Obama to stand by his insistence that the use of such weapons crossed a "red line". Syrian state TV said there was "no truth whatsoever" in the allegations.

The US, UK, France, Germany, the EU and the Arab League are among those who have publicly called for UN chemical weapons investigators – who arrived in Damascus three days ago to look into previous allegations of such attacks – to be dispatched to the scene of what may be one of the deadliest incidents of Syria's two-year-old civil war.

A White House spokesman said the US was "deeply concerned" by the reports and urged the Syrian government and opposition to provide immediate access to investigators. The Security Council is to meet in New York on Wednesday at 3pm (8pm BST).

Britain's foreign secretary, William Hague said that if the reports were confirmed, they would mark a "shocking escalation" in the use of such weapons.

Syrian opposition sources said rockets with toxic agents hit the Ghouta area, east of the capital, where there is a rebel presence. The opposition Sham news network reported that the nerve agent sarin had been used.

Numerous graphic videos have been posted online, showing people with a range of symptoms including constricted pupils, difficulty breathing, foaming at the mouth, and shaking or fits. The patients, many of them children sprawled on tiled floors and piled on hospital beds, have been stripped down seemingly in an effort to free them of the toxic substances on their clothes.

Other videos show lines of dead bodies on the floor. Experts have said that they appear to show evidence of use of a poisonous gas but it is impossible to tell which one without further investigation.

Bayan Baker, a nurse at Douma Emergency Collection facility in a suburb of Damascus, said the death toll, collated from medical centres in the region, was 213, Reuters reported.

"Many of the casualties are women and children," she said. "They arrived with their pupil dilated, cold limbs and foam in their mouths. The doctors say these are typical symptoms of nerve gas victims."

One photograph purportedly taken by activists in Douma showed the bodies of at least 16 children and three adults, one wearing combat fatigues, laid on the floor of a room in a medical facility, where the bodies had been collected.

In a statement hours after the alleged incident, Hague said: "I am deeply concerned by reports that hundreds of people, including children, have been killed in air strikes and a chemical weapons attack on rebel-held areas near Damascus.

"These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria."

He added: "Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account.

"I call on the Syrian government to allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use. The UK will be raising this incident at the UN security council."

A team of UN weapons inspectors is already in Damascus to investigate claims of the use of chemical weapons in March so it should, in theory, be possible to look at the Ghouta case. The Assad government would have to approve any such investigation.

International attention on Syria, which was already fading, has been further diverted in recent weeks by the crisis in Egypt.

Ghouta has been the focus of intense clashes between government and Hezbollah soldiers and rebel forces. Both sides are anxious to secure the outskirts of Damascus near the border with Lebanon to the country's north, where Jabhat al-Nusra and other extremist groups are concentrated.

According to the Syrian Arab News Agency, the area is an important link in the chain of the weapons supply route from Jordan. Around 1.5 million people in eastern Ghouta have been trapped in an intermittent siege and cut off from basic supplies since their liberation by the Free Syrian Army in early 2012.