United Nations investigators will examine soil samples collected by western intelligence agencies and enter Syrian refugee camps in an effort to assess claims that the Assad regime has used sarin gas against its opponents.

Proof of sarin use would increase pressure on the Obama administration which, after the wars in Iraq and Afghanistan, is deeply reluctant to intervene in what could be another protracted and unwinnable conflict.

The White House has long claimed the use of chemical weapons by the Syrian regime would represent a "red line", but has so far been reluctant to follow Britian, France and Israel, who claim to have evidence of chemical weapons use in Aleppo and Homs.

Syria has prevented the UN investigation from being carried out inside the country, but Jeffrey Feltman, UN under-secretary for political affairs, made it clear on Wednesday that refusal would not prevent an inquiry from being carried out.

"The secretary-general [Ban Ki-moon]'s position is that, at this time, the mission should investigate the allegations pertaining to incidents in Aleppo and Homs. While awaiting access to the Syrian territory, the experts of the mission are studying the information on the alleged incidents of the use of chemical weapons provided to them by member states," Feltman told the security council.

"They remain ready to deploy to Syria within 24 to 48 hours following the Syrian government's acceptance of the modality and the scope of the mission." He deliberately did not name the member countries but they include Britain, France and Israel.

It is understood that as well as visiting refugee camps and potentially taking hair and other biological samples from survivors of alleged chemical attacks, UN investigators will also analyse soil samples in the possession of British and French intelligence agencies.

'Obama administration is demanding a lot of direct evidence'

British and French officials believe there is incontrovertible evidence that the Syrian government has used sarin nerve gas, though only on a small scale.

"There is no doubt that sarin has been used but we are not talking about Halabja," one Whitehall source said, referring to the 1988 gas attack by Saddam Hussein's forces against Iraqi Kurds, in which up to 5,000 were killed by a combination of Sarin, other nerve agents and mustard gas. "We are talking about use in small areas and small groups of people."

British officials are adamant that the source of the sarin was the government and that the exposure of Syrian army troops in the town of Khan al-Asal on March 19, as claimed by Damascus, was the result of "friendly fire", a government shell that had gone astray, rather than a rebel attack.

A French official said that it would be up to the UN investigators " to establish evidence that will allow to confirm the use of chemical weapons".

The official added: "we deplore that the regime is seeking to impose unacceptable conditions on the UN for the deployment of the team … If the regime does not come back on its decision, it will bear the entire responsibility of the team's failure. That would be a renewed proof of its duplicity."

The British and French claims, backed up publicly by an Israeli general, have opened a rift with the US, which has been far more cautious about its claims, privately questioning the strength of their allies' case.

On a visit to Egypt, the US defence secretary Chuck Hagel faced intensive questioning from American reporters. He denied that US credibility was at risk over Obama's failure to respond over chemical weapons, and played down the conclusions of British, French and Israeli intelligence.

He said: "Suspicions are one thing. Evidence is another ... And that's not at all questioning other nations' intelligence, but the United States relies on its own intelligence. So until I can see that intelligence, I really don't have anything else to say."

A group of Arab foreign ministers is due in Washington on Monday and are expected to meet the secretary of state, John Kerry. That meeting may prove crucial in formulating Washington's next step.

British officials argue the real difference between London and Paris on one side and Washington on the other is political rather than factual, as President Obama had previously taken the lead in making the use of chemical weapons a red line.

Syrian citizens inspect a damaged car next to the municipal building in Damascus on Thursday. Photograph: Sana Handout/EPA

The stand-off with Syria has echoes of the run-up to the Iraq war. The Obama administration is desperate to avoid a repeat of that debacle, not least because of the way intelligence was discredited at that time, with no weapons of mass destruction found after the 2003 invasion.

Obama said last month that if it it was found that such weapons were used, the Assad regime would have crossed the red line. The White House has since rowed back, with the New York Times quoting aides saying he only meant major chemical weapons such as those used by former Iraqi dictator Saddam Hussein against the Kurds.

The Obama administration is seeking to make a distinction between lethal chemical weapons and ones that only have a temporary effect. But sarin, which attacks the nervous system, is lethal. The US intelligence assessment is that Syria has a stockpile of sarin, VX and mustard gas.

The UN investigations team is in Cyprus awaiting entry into Syria. Assad initially said the team would be allowed in but later blocked them.

A security council source said: "While we would like the investigation to go ahead in Syria ... we are hopeful that the investigation team will still be able to undertake elements of the investigation even without access to Syria. This could include conducting interviews in refugee camps."

The Russian foreign minister, Sergei Lavrov, was quoted by the Russian news agency Novosti warning that its ally Syria should not face a repetition of "Iraq scenario".

Tony Cordesman, of the Center for Strategic and International Studies in Washington, said the White House's reluctance to become involved was understandable. "It is far easier to talk about reacting to Syria's possible use of chemical weapons than do something meaningful. First, there needs to be firm evidence. The US cannot commit an act of war on the basis of outside claims or 'evidence' that is suspect and may be politically motivated – particularly after invading Iraq in search of weapons of mass destruction that did not exist."

Cordesman said an attack on Syrian targets would represent a serious escalation. "It would take a massive ground intervention to seize and destroy Syria's stocks of chemical weapons, if they have not been partially dispersed to the point where such a mission is impossible. This means fighting the way in, fighting long enough seize and destroy the weapons, and fighting on the way out."

But analysts question the wisdom of western governments laying down public ultimatums about the use of chemical weapons. Bruno Tertrais, senior research fellow at the Foundation for Strategic Research in Paris, said: "The downside is that it implicitly signals to the Syrian government that anything below that line is not unacceptable."

Security officials also caution that it is not clear what western forces could do even if the use of chemical weapons was proved or found to be imminent. Bombing Assad's chemical arsenal would disperse the chemicals and nerve agents with potentially disastrous consequences, while any special forces deployed to seize and secure such weapons would immediately become stationary targets for Islamist groups now fighting with the rebels.

"My immediate reaction is to urge caution," said Dina Esfandiari of the International Institute of Strategic Studies in London. "If these claims are based on photographs and soil samples taken some time after the alleged attacks, I don't see how could you possibly be sure. And without definite proof how can you assert that it is the regime and not the other side?"

Andrew Tabler, a Syria specialist at the Washington Institute of Near East Policy and author In the Lion's Den: An Eyewitness Account of Washington's Battle with Syria, warned that the longer the Obama administration delays in enforcing its red line, the worse it will be later. "I think in the end we will be drawn into this conflict and it will be ten times more expensive than now. Everything will be worse," he said.

"The Obama administration is demanding a lot of direct evidence before it reacts. It just shows the reluctance of the Obama administration to enforce its red line. The Assad regime is pushing and testing the US and the US is not reacting, except verbally," Tabler said.

He predicted the country would disintegrate into three pieces, with terrorists potentially active in all three. "That would be a policy failure by any stretch of the imagination," he said.