Opposition activists have said the Syrian government has used chemical weapons during an attack outside Damascus, killing hundreds. The Arab League has called on a UN team of experts, already in Syria, to investigate.
Several pro-opposition groups have accused President Bashar al-Assad's government forces of carrying out a "poisonous gas" attack during intensified shelling near the capital on Wednesday.
The attacks took place in the Ghouta region on the outskirts of Damascus, said the Syrian Observatory for Human Rights, the Britain-based, pro-opposition monitoring group.
The Syrian government has denied the reports. State television quoted a source as saying there was "no truth whatsoever" to the allegations. The source said the reports were meant to distract a United Nations chemical weapons team from their mission.
A UN team of experts is currently in Syria investigating claims that chemical weapons were used in attacks earlier this year. Both the government and opposition forces blame each other for the attacks.
Arab League chief Nabil al-Arabi urged the UN team in a statement to "go immediately to Eastern Ghouta to see the reality of the situation and investigate the circumstances of this crime."
British Foreign Secretary William Hague said Britain will refer the reported chemical weapons use to the UN Security Council. He also called on the Syrian government to allow the UN team immediate access to the area for investigation.
"I am deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus," Hague said.
The United States and several European countries that have aligned themselves with the Syrian opposition have said they believe Assad's government has used chemical weapons in the past. Washington says the government's alleged use of the weapons crossed a so-called "red line" that warranted international military aid to the opposition. Russia, an ally of Assad's government, has accused rebel fighters, rather than the Syrian regime, of using chemical weapons.
Varying death tolls
The Dubai-based Al-Arabiya news channel reported that at least 500 people were killed in the shelling. The Syrian Local Coordination Committees, a group that document violence across Syria, put the number of dead at more than 600, reporting the "brutal use of toxic gas by the criminal regime in parts of Western Ghouta."
A leading opponent of Assad, George Sabra, has even put the death toll at 1,300, saying the alleged massacre was a "coup de grace that kills all hopes for a political solution in Syria."
Activists posted pictures online of a makeshift hospital with people, including children, being treated on the floor.
Another activist group, the Syrian Revolution General Commission, posted videos on YouTube showing what it called "a terrible massacre committed by regime forces with toxic gas, leaving dozens of martyrs and wounded."
The news agency Reuters reported that Bayan Baker, a nurse at Douma Emergency Collection facility, said the death toll from the attack was 213.
"Many of the casualties are women and children. They arrived with their pupils dilated, cold limbs and foam in their mouths. The doctors say these are typical symptoms of nerve gas victims," she said.
None of the casualty reports could be independently verified.
hc/dr (Reuters, AFP, AP, dpa)