The UN Security Council has held an emergency meeting to discuss an alleged deadly gas attack in Syria.
Syria’s opposition accused the government on Wednesday of using chemical arms to strike rebel-held areas in the suburbs of Damascus, killing hundreds of people.
Videos distributed by activists, which could not be independently verified, showed medics attending to asphyxiating children and hospitals being overwhelmed. More footage showed dozens of people laid out on the ground, with no visible wounds or trauma.
There have been conflicting reports on the death toll. The Syrian Observatory for Human Rights, a Britain-based watchdog group, said that the attack killed at least 100 people.
However, Salim Idriss, the military chief of the Free Syrian Army, told Al Jazeera that at least 1,600 were killed and hundreds more injured.
The Syrian National Coalition's George Sabra said that more than 1,300 people had been killed in what he described as a "coup de grace that kills all hopes for a political solution in Syria".
"The Syrian regime is mocking the UN and the great powers when it strikes targets near Damascus, while the [UN weapons inspectors] are just a few steps away," he said.
The attack coincided with the visit by a 20-member UN chemical weapons team to Syria to investigate three sites where chemical weapons attacks allegedly occurred over the past year.
The Syrian armed forces strongly denied the usage of chemical weapons, and state television said the accusations were fabricated to distract the UN investigators.
The UN said its chief chemical weapons inspector Ake Sellstrom was in discussions with the Syrian government over the alleged attack.
The Security Council met after several Western and regional powers called for the UN team to be dispatched to the scene.
The European Union condemned the suspected use of chemical weapons as "totally unacceptable".
“We are awaiting further information about this but, if verified, this 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,” said British Foreign Secretary William Hague.
In Cairo, the Arab League also urged the UN inspectors to visit the site of the alleged attack immediately.
Ralf Trapp, a chemical weapons expert, told Al Jazeera that with the UN team being present in the country, a very effective investigation could be conducted.
"It could be conducted very swiftly, because you are now in a time frame [of a few hours or days after the attack] so you can find the actual agent or degradation products of the agent, in biological samples and also in the environment," he said, speaking from France.
He said the symptoms that people in the videos he viewed showed were consistent with the possible use of a chemical agent.
However, Paula Vanninen, the director of Verifin, the Finnish Institute for Verification of the Chemical Weapons Convention, told Al Jazeera that "some of those people were shaking and could have gotten the nerve agent exposure but for others it’s quite difficult to say from the video what has caused their death".
"But you can see that they couldn’t breathe. Something has caused that – they were killed by not being able to breathe anymore," she added.
Russia's Foreign Ministry spokesman Alexander Lukashevich said circumstances around the reports, including the presence of UN inspectors in the country, suggested that attack could be a provocation by the opposition.
"All this cannot but suggest that once again we are dealing with a pre-planned provocation. This is supported by the fact that the criminal act was committed near Damascus at the very moment when a mission of UN experts had successfully started their work of investigating allegations of the possible use of chemical weapons there," Lukashevich said in a statement.
Activists said rockets with chemical agents hit the Damascus suburbs of Ain Tarma, Zamalka and Jobar during fierce pre-dawn bombardment by government forces.
The Damascus Media Office monitoring centre, citing figures compiled from medical clinics, said 150 bodies were counted in Hammouriya, 100 in Kfar Batna, 67 in Saqba, 61 in Douma, 76 in Mouadamiya and 40 elsewhere in Damascus suburbs.
They added that at least 90 percent of them were killed by gas and the rest by shelling.
Syria is said to have one of the world's largest stockpiles of chemical weapons, including mustard gas and the nerve agent sarin. The government refuses to confirm or deny it possesses such weapons.
Rebels and the government have accused each other of using chemical weapons in attacks during the country's civil war.
In June, the US said it had conclusive evidence that Bashar al-Assad's regime used such arms against opposition forces.