UN Security Council members are seeking "clarity" on opposition claims that Syrian troops used chemical weapons in an attack near Damascus, Argentina's UN envoy, the council president, said Wednesday.

UN Secretary General Ban Ki-moon's office said earlier he was "shocked" by the reports and that UN weapons experts in Syria to probe previous allegations were in discussions with Damascus.

"There must be clarity on what happened and the situation must be followed carefully," Argentina's envoy Maria Cristina Perceval said after the council met behind closed doors.

Council members -- who were briefed by Deputy Secretary General Jan Eliasson -- "welcomed the determination of the secretary general to ensure a thorough, impartial and prompt investigation," she said.

Perceval, whose country is presiding over the Council for the month of August, said members had expressed "strong concern" about the allegations made by the Syrian opposition.

There was a "strong call for a cessation of hostilities and for a ceasefire," she said.

The main Syrian opposition group claims as many as 1,300 people were killed in a chemical weapons attack Wednesday on rebel areas near Damascus.

Videos distributed by activists, the authenticity of which could not immediately be verified, showed medics attending to suffocating children and hospitals being overwhelmed.

"All Council members agree that any use of chemical weapons by any side under any circumstances is a violation of international law," Perceval said, highlighting the need for "immediate humanitarian assistance to the victims."

The council did not issue a formal statement at the end of the meeting. Diplomats said Russia and China -- who have repeatedly backed Syria since the start of the crisis -- had opposed the adoption of a statement.

Council members France, Britain, the United States, Luxembourg and South Korea had requested Wednesday's meeting.

Several council members including the United States and France have asked that the team of UN inspectors be dispatched immediately to the scene to investigate.

Washington demanded that Syria provide immediate access to the site, while Russia, a close ally of the Syrian government, called the opposition claims a "provocation."

Britain, France, Germany and the United States sent a formal request for an investigation of the incident to Ban's office.

Diplomats said the joint letter cited "credible reports of the use of chemical weapons."

"We urge you to do all you can to ensure that the mission has urgent access to all relevant sites and sources of information," the letter said.

Earlier, a UN diplomat however said it would be difficult for the UN experts to investigate the incident because the alleged attack site was not one of three where the Syrian regime had agreed to UN inspections.

That means that the head of the UN experts in Syria, Ake Sellstrom of Sweden, must negotiate access to the new site with Syrian authorities, the diplomat said.

A UN statement said Sellstrom's team was "following the current situation in Syria carefully, and remains fully engaged in the investigation process that is mandated by the Secretary General.

"Professor Sellstrom is in discussions with the Syrian Government on all issues pertaining to the alleged use of chemical weapons, including this most recent reported incident."

Eliasson said the UN hoped the experts "will be given access to the area by the government" but acknowledged that "the security situation right now does not allow such access."

"This represented a serious escalation," Eliasson said. "There is a great need for a cessation of hostilities in this particular area, and in general."