UNITED NATIONS — The UN Security Council called Wednesday for “a thorough, impartial and prompt investigation” of the latest allegation of chemical weapons use in Syria.

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.

UN deputy spokesman Eduardo del Buey said earlier that the secretary-general was “shocked” at Wednesday’s alleged use of chemical weapons and wants all reported incidents investigated.

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During a two-hour closed-door meeting, diplomats said Russia and China, which support the Syrian government, blocked a stronger press statement supported by Britain, France, the United States and others.

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, speaking on condition of anonymity because the discussions were private.

The proposed stronger statement, according to reports, would have called on Syria to allow UN inspectors to look into Wednesday’s alleged attack.

A Russian official called the alleged attack a “planned provocation on the part of the Syrian opposition.”

Argentina’s UN Ambassador Maria Cristina Perceval, the current council president, told reporters that there was “strong concern” about the latest allegations of alleged chemical weapons use “and a general sense that there must be clarity on what happened.”

Syrian anti-government activists accused President Bashar Assad’s regime of carrying out a toxic gas attack that killed at least 100 people, including many children as they slept. The government denied using chemical weapons.

Perceval said council members “welcomed the determination of the secretary-general 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.”

German Chancellor Angela Merkel said Wednesday that if reports of the chemical attack are true, it is a “horrific crime.” She rejected, however, the idea that the EU should speed up military aid to the Syrian rebels.

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.

Before the council meeting, a letter drafted by Britain and France was sent to the secretary-general requesting that the team launch “an urgent investigation … as expeditiously as possible” into Wednesday’s incident as well.

Britain’s deputy UN ambassador Philip Parham said about 35 countries signed the letter, including the United States.

The letter, obtained by The Associated Press, cited “credible reports of the use of chemical weapons” and urged the secretary-general “to do all you can to ensure that the mission has urgent access to all relevant sites and sources of information.”

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 White House spokesman said the US supported a UN investigation and a Security Council debate.

Under the terms of an agreement between the UN and the Syrian government, the UN team will investigate an alleged chemical weapons attack on March 19 on the village of Khan al Assal outside the city of Aleppo, which was captured by the rebels last month. It will investigate two other sites of alleged attacks, which are being kept secret for security reasons.

Chemical weapons experts have raised doubts about whether the chemical weapons experts will find anything at the three sites because the alleged attacks took place months ago.

But if chemical weapons were used Wednesday, a prompt investigation would be much more likely to produce evidence.

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.

Diplomats cautioned that the mandate for the current investigation is limited.

The UN team will report on whether chemical weapons were used, and if so which ones, but it will not determine the responsibility for an attack. This has led some commentators to question the value of the investigation.

On June 13, the United States said it had conclusive evidence that Assad’s regime had used chemical weapons against opposition forces. That crossed what President Barack Obama had called a “red line” and prompted a US decision to send arms and ammunition to the opposition, though reports say none have arrived yet.