Ban said the inspectors, who are investigating the chemical weapons attack last week, would need a total of four days to carry out their site visits and then further time to analyse their findings.
He spoke as a 90-minute meeting of the National Security Council (NSC), devoted to discussing the options for targeted attacks against Syria, broke up in London before a debate and vote in the House of Commons on Thursday on government plans to respond with force to Syria’s use of chemical weapons.
Sources in London and Washington have been suggesting that a limited attack could take place before the end of the week, but Cameron’s desire to show that he is not ignoring the UN could put that timetable in jeopardy.
The Labour party said it would only support the government if the matter went to the UN security council and if the evidence from the weapons inspectors was considered.
At a news conference in The Hague, Ban said the inspectors needed four days to complete their work 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.”
Britain has announced that it will present a resolution at a meeting of the five permanent members of the security council on Wednesday afternoon authorising military intervention. Russia and China are expected to block it, but Cameron needs to be seen to be trying to secure UN approval to maximise his chances in Thursday’s vote.
But Britain may struggle to get a swift response. The Interfax news agency quoted a senior Russian diplomat as saying that the security council should wait for a report from the weapons inspectors before deciding what to do.
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 those same 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.”
A Downing Street spokeswoman said: “Britain has drafted a resolution condemning the attack by the Assad regime, and authorising all necessary measures under chapter 7 of the UN charter to protect civilians from chemical weapons.
“The resolution will be put forward at a meeting of the five permanent members of the security council later today in New York.”