Syrian troops and opposition fighters have clashed during fierce battles in suburbs of the Syrian capital where the opposition claims a chemical weapons attack this week killed more than 130 people.
As the government pursued its offensive on the rebel-held eastern suburbs for a third day, United Nations Secretary-General Ban Ki-moon urged the Syrian government to allow a UN team now in Damascus to swiftly investigate the alleged chemical weapons attack.
UN deputy spokesman Eduardo del Buey said Mr Ban has been in touch with world leaders since Wednesday and is sending UN disarmament chief Angela Kane to Damascus to press for an investigation.
Syrian opposition figures and activists have reported death tolls from Wednesday's attack ranging from 136 to 1,300. If confirmed, even the most conservative tally would make it the deadliest alleged chemical attack in Syria's civil war.
The Britain-based Syrian Observatory for Human Rights and Syria-based activist Mohammed Abdullah said government warplanes and artillery were pummeling different parts of the Damascus suburbs today, including the areas allegedly hit by toxic gas this week.
US President Barack Obama called the possible chemical attack a "big event of grave concern." In an interview broadcast today on CNN, he said the event was "very troublesome" and was going to "require America's attention."
Just more than a year ago, Mr Obama warned that the use of chemical weapons in Syria would be a "red line" for the US and carry "enormous consequences." Washington then said in June that it had conclusive evidence that President Bashar Assad's regime had used chemical weapons against opposition forces, prompting a US decision to begin arming rebel groups, although that has not happened yet.
While the US is providing humanitarian support and financial aid to the opposition, the Obama administration has made clear its desire not to get directly involved militarily in Syria's bloody conflict. But that position is coming under increasing pressure following Wednesday's alleged gas attack as both the Syrian opposition and many of their Western allies clamor for a forceful international response if chemical weapons were indeed employed.
Already, the US Britain and France have called for the UN team in the country to be granted immediate access to the locations hit on Wednesday.
Russia, a close ally of the Assad regime, called today for an independent probe by UN experts, but did not specify where the inspectors already in the country should immediately visit the areas hit.
The Russian Foreign Ministry also said that Minister Sergey Lavrov and US Secretary of State John Kerry had discussed the situation by telephone yesterday, and concluded that they had a "mutual interest" in calling for the UN investigation. The ministry said Russia had called for Mr Assad's embattled government to co-operate with a probe, but questions remained about the willingness of the opposition, "which must secure safe access of the mission to the location of the incident."
At the same time, Moscow cast doubt on whether a chemical attack even occurred, saying "new evidence that is starting to emerge increasingly shows that this criminal act was clearly a provocation."
The UN team has been in Damascus since Sunday to investigate three sites where past chemical weapons attacks allegedly occurred: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations kept secret for security reasons. It took months of negotiations between the UN and the Syrian government to agree on the limited mandate of the probe.
Deputy Prime Minister Qadri Jamil said he was personally in favour of a fair, transparent international delegation to investigate the most recent incident in Ghouta. But he said that would require a new agreement between the government and the UN and that the conditions for such a delegation would need to be studied.
The timing of an investigation is of acute importance because it becomes increasingly difficult to reliably detect traces of what chemical, if any, was used in an attack as time passes. An immediate probe would also allow inspectors to collect blood and soil themselves and transfer it from the site to a laboratory without intermediaries, guaranteeing the chain of custody of the evidence and the integrity of the investigation.
In neighbouring Lebanon, the army said in a statement that troops have captured a truck with "large amounts" of gas masks in the southeastern village of Kfeir near the border with Syria. A Lebanese army general said an investigation is under way about the masks.