Talks could take days, foreign secretary says as pressure mounts on west to let weapons inspectors finish their job

William Hague has played down the prospects of an imminent attack on Syria, saying talks about the crisis could go on at the United Nations "over the coming days".

Speaking after a meeting of Britain's National Security Council, the foreign secretary said the UN security council should "shoulder its responsibility" over Syria; if it failed to do so, however, Britain and and its allies would act on their own.

"This is the first use of chemical warfare in the 21st century," he said. "It has to be unacceptable. We have to confront something that is a war crime, something that is a crime against humanity."

Britain and the other four permanent members of the security council have begun talks in New York on a resolution proposed by Britain that would authorise the use of military force in Syria as a response to its use of chemical weapons last week.

Russia and China are firmly opposed to a security council resolution allowing force, and British government officials admit their initiative is likely to fail. But Labour said on Wednesday it would not support the government in Thursday's Commons vote unless ministers had tried to win security council agreement.

Hague said: "It is time the United Nations security council shoulders its responsibility on Syria, which for the last two and a half years it has failed to do.

"We have put forward to them a draft resolution which condemns the use of chemical weapons, which demands that the Assad regime cease to use such weapons and which resolves to do what is necessary to alleviate the suffering of the Syrian people affected by chemical weapons attacks and to try to prevent the further use of the chemical stockpiles of the Assad regime.

"I expect there will be further discussions in New York over the coming days but we have started those discussions about a UN resolution, because by far the best thing would be if the United Nations could be united, unlikely as that seems in the face of vetoes from Russia and China that we have had in the past. But we have to try to do that.

"We are clear that … if there isn't agreement at the United Nations, then we still have a responsibility; we and other nations still have a responsibility."

Hague also said he thought it would be a mistake for military action to be delayed too long. "It's very important not to take so long to respond that people confuse what the eventual response is about," he said.

Sources in London and Washington have been suggesting that a limited attack could take place before the end of the week, but Britain's desire to show that it is not ignoring the UN could put that timetable in jeopardy.

Earlier, at a news conference in The Hague, Ban Ki-moon, the UN secretary general, said UN weapons inspectors needed four more days to complete their work investigating the chemical weapons attacks in Damascus; they are now on their second day.

"They are working very hard,under very, very dangerous circumstances," he said. "Let them conclude their work for four days, and then we will have to analyse scientifically with experts. And then I think we will have to report to the security council for any actions."

The British government believes that, even without a new security council resolution, it would have the legal authority to use force to respond to the use of chemical weapons. But on Wednesday Ban's special envoy for Syria, the Algerian diplomat Lakhdar Brahimi, warned that "international law is clear" about security council authorisation being required for any military action.

After the meeting of the NSC, Cameron posted a message on Twitter about its conclusion. "The NSC agreed unanimously that the use of chemical weapons by Assad was unacceptable – and the world should not stand by," he wrote.

In Syria, Bashar al-Assad's regime stoked up its rhetoric against the west, with Faisal Maqdad, the deputy foreign minister, accusing Britain, the US and France of helping "terrorists" use chemical weapons in the country, and saying that Islamist extremists could soon use those weapons "against the people of Europe".

Ban spoke to reporters after delivering a speech at the Peace Palace in The Hague in which he appealed to the west to "give peace a chance" and "give diplomacy a chance" in Syria.

Although Downing Street announced that it was presenting a resolution at the UN shortly after the Labour party demanded UN involvement, Downing Street sources said approaching the UN had always been part of the government's plan, and denied Labour had bounced Cameron into acting.

One Liberal Democrat source said the deputy prime minister, Nick Clegg, had been particularly keen to take the matter to the UN, and that when Cameron, Clegg and the Labour leader, Ed Miliband, met to discuss Syria on Tuesday afternoon Clegg was the first person to raise the importance of trying to secure UN support.

Previously the government has in public played down the need for a debate at the security council, where Russia and China have been staunch opponents of anti-Assad initiatives. But, in one tweet on the subject, Cameron said he wanted the UN to "live up to its responsibilities on Syria".

Earlier, Downing Street confirmed that Cameron spoke to the US president, Barack Obama, on Tuesday night. Although Downing Street said Cameron and Obama had not yet agreed on the "specific nature" of their response to the use of chemical weapons in Syria, it is understood that they are planning limited missile attacks before the end of the week.

Cameron has recalled parliament to allow MPs to vote on the matter on Thursday. On Tuesday afternoon, after Ed Miliband had met Cameron to discuss the matter, Labour indicated that it would be willing to support the government, provided military action was legal and proportionate. But early on Wednesday morning Labour said it was making its support for the government dependent on new conditions.

A party spokesman said: "We have made it clear that we want to see a clear legal basis for any action. As part of the legal justification, Labour is seeking the direct involvement of the United Nations through the evidence of the weapons inspectors and consideration by the security council."

This raised the possibility that Labour could refuse to back the government's motion on Thursday, perhaps voting for its own motion instead, although the party said it would not take a decision until the text of the government's motion was available, later on Wednesday.

The British and American governments have until now dismissed suggestions that military action should be delayed until the UN weapons inspectors in Damascus have reported, arguing that it is already obvious that chemical weapons were used and that the inspectors' report will not say which side was responsible for their deployment.

Cameron's move goes some way to meeting Labour concerns. In response, a Labour source said: "This is one necessary step. We will continue to scrutinise any proposed action to ensure there is a proper legal base."