The United Nations team of chemical weapons inspectors who spent almost two weeks investigating the attack in the Damascus suburb of Ghouta in which hundreds of civilians died, have returned to their headquarters at The Hague in the Netherlands. There, they are beginning scientific analysis of samples designed to give a categoric answer to the question of whether a war crime was committed.

The team is now racing against the clock to complete the analysis, which is being carried out in two unidentified laboratories in Europe. The UN has been granted some breathing space by President Obama's surprise announcement that he will call a vote in Congress over military intervention, but the pressure remains intense for the team to report before any unilateral US air strikes are launched.

The UN secretary general, Ban Ki-Moon, has asked the inspectors to expedite the work on the samples but has also made it clear that he will not be bullied into curtailing proper scientific processes. On Saturday, Ban promised through his official spokesperson that "whatever can be done to speed up the process is being done".

As the drum beat of possible unilateral military action by the US backed by France intensifies, the UN is perilously close to appearing irrelevant in the crisis. The security council is paralysed by disagreement between its five permanent members – the US and France on the side of military intervention, Russia and China opposed, and the UK staying on the sidelines after parliament voted against any action.

The UN inspection team has also been hampered by the mandate set for it in Syria by the general assembly and endorsed in a resolution from the security council. That instructed the dozen-strong team of chemical weapons experts and medical specialists to ascertain whether or not chemical weapons – probably nerve gas – had been deployed in Ghouta on 21 August.

But the mandate did not include any attempt to identify who carried out the attack or to apportion blame. Both sides in the conflict – the Syrian regime and rebel forces – have accused the other side of responsibility for the outrage.

"The mandate is the mandate," said Martin Nesirky, Ban's official spokesperson, when asked by reporters why the UN had not expanded its mission to include the question of culpability. He added: "The aim of the game here, the mandate, is very clear: to ascertain whether chemical weapons were used, not by whom."

The UN's role in attempting to defuse the threat of military action and steer all parties towards a diplomatic solution is also being undermined by the Obama administration, which has made it clear that it is prepared to go ahead with air strikes even before the inspection team produces its report. On Saturday, Obama said he was "comfortable" about ordering airstrikes without UN authorisation, on the grounds that the UN security council was "entirely paralysed" and incapable of providing leadership.

On Friday, the secretary of state, John Kerry, was almost openly disparaging of the UN effort, saying there was nothing that the inspection team could tell US intelligence experts that they did not already know.

Kerry said the US had gathered evidence that forces loyal to the Syrian government had spent three days preparing for the Ghouta attack and had launched chemical weapons via rockets sent from regime-controlled areas of the city. "All of these things we know, the American intelligence community has high confidence," he said.

Despite such undermining from Washington, the UN is trying to shore up its shaky position in the centre of the crisis. In a briefing at the UN building in New York on Saturday, Nesirky said the UN mission was "uniquely capable of establishing in an impartial and credible manner the facts of any chemical weapons used, based on evidence from the ground."

Ban spent Saturday morning locked in discussions with the UN disarmament chief, Angela Kane, who returned to New York from Damascus. The secretary general is also in regular contact with Dr Åke Sellström, the Swedish scientist who led the inspection team in Syria.

The secretary general has pledged that once they have completed their report into the Ghouta attack, the UN inspectors will return to Syria to investigate all other allegations of chemical weapons use in the country's civil war. The Syrian government has countered accusations that it is guilty of deploying chemical weapons against international law by accusing rebels of attacking its soldiers with nerve gas.

Though the chemical weapons inspectors are now safely out of Syria, there are still more than 1,000 UN workers still within the country. Nesirky said reports that the inspectors' departure had opened a window for US airstrikes were "grotesque and an affront to the more than 1,000 UN staff on the ground in Syria delivering humanitarian aid and who will continue to deliver humanitarian aid." © Guardian News and Media 2013