UNITED NATIONS - The UN Security Council called for "a thorough, impartial and prompt investigation" of the latest allegation of chemical weapon use in Syria when the 15-member body met in an emergency session late Wednesday during which Pakistan have rejected claims about Syrian government's involvement in the attack.

The Council backed Secretary-General Ban Ki-moon’s determination to ensure a “thorough investigation” of the alleged attack on the eastern suburbs of Damascus that killed at least 100 people.

"There is a strong concern among council members about the allegations and a general sense that there must be clarity on what happened and the situation must be followed closely,” Argentina’s UN ambassador, Maria Cristina Perceval, the current Security Council president, told reporters after a closed-door meeting of the UN's power centre.

The United States, Britain and France are among around 35 countries that called for chief UN investigator Ake Sellstrom, whose team is currently in Syria, to investigate the incident as soon as possible. Syrian anti-government activists accused President Bashar Assad’s government of carrying out a toxic gas attack. The government denied using chemical weapons.

On its part, Pakistan fully backed the move for an impartial investigation into the tragic incident, but questioned claims about the Syrian government's involvement in the gas attack, according to Council sources. "It seems totally counter-intuitive that the Syrian Government would use chemical weapons, especially after allowing the UN team, led by Ake Sellstrom, into the country to investigate the use of such weapons," Pakistan UN Ambassador Masood Khan told the Security Council. "It would be a most brazen and defiant move on part of the Syrian Government to implicate itself under the glare of international spotlight."

Noting Syrian Government's denial of the use of gas and the opposition activists' insistance that it has, the Pakistani envoy said it was extremely important to determine facts through an impartial investigation.  "We have heard claims and counter-claims in the past too from Government and opposition," Masood Khan said.

, adding: "The veracity of these claims has to be ascertained beyond a shadow of doubt."

During the private Council meeting, Russia and China, which support the Syrian government, blocked a stronger press statement supported by Britain, France, the United States and others, according to western diplomats who declined to be named. But Russia and China agreed that the council president could sum up the session with "press elements" - close to the weakest response from the UN's most powerful body, the diplomats said,

Ambassador Perceval, the Council's president,  said Council members “welcomed the determination of the secretary-general (Ban Ki-moon) to ensure a thorough, impartial and prompt investigation.”

“All council members agreed that any use of chemical weapons by any side under any circumstances is a violation of international law,” she said. “There was also an agreement for a strong call for a cessation of hostilities and a cease-fire ... (and) the need for immediate humanitarian assistance to the victims.”

The attack coincided with the visit to Syria by a 20-member UN chemical weapons team which only has a mandate to investigate three previous allegations of chemical weapons use.

UN Deputy Secretary-General Jan Eliasson, who briefed the Council, expressed hope that the Syrian government will give the team access to the site as soon as possible, though he cautioned that “the security situation right now does not allow such access.”

This represents, no matter what conclusions are, a serious escalation with grave humanitarian consequences and human consequences,” Eliasson stressed.

He called for a cessation of hostilities not only in the area of the alleged attack but throughout Syria, where the UN says more than 100,000 people have been killed in the 2 1/2 year conflict.

“What this incident has shown is, of course, that we must contain this conflict,” Eliasson said. “We have already seen effects on the regional implications, and now, the possibility of the use of chemical weapons which is to be investigated.”

A UN spokesman, Eduardo del Buey, said the head of the UN team, Swedish professor Ake 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.”

He said the secretary-general was aware that a number of member states, the Arab League and the European Union have expressed “grave concern” about the latest alleged attack.

“The secretary-general reaffirms his determination to ensure a thorough investigation of the reported alleged incidents that are brought to his attention by member states,” del Buey said.

Some diplomats cautioned that the mandate for the current investigation is limited.

"The current UN investigations team is not mandated to to affix responsibility but to establish whether or not chemical weapons have indeed been used," Pakistan's Ambassador Masood Khan told the meeting, according to diplomatic sources.

"Once the use of chemical weapons is confirmed, a new mechanism could be created by the United Nations with the mandate to assign responsibility and recommend further measures," he said.

"A credible, professional and objective evaluation of the alleged claims of the past is of utmost importance." He urged the Syrian Government to cooperate fully with the UN team.

"The right course of action is that the UN team on the ground be authorized to visit the site to establish the release of Sarin gas as a weapon."