UN inspectors have collected "considerable" evidence on a suspected chemical weapon attack in Syria and will brief UN leader Ban Ki-moon soon after they leave on Saturday, a spokesman said.

While the major powers wrangle over a possible military strike on Syria, samples collected in the country will be sent to laboratories across Europe and the analyses could take more than a week, UN spokesman Farhan Haq told reporters on Thursday.

Ban cut short a trip to Europe to return to New York on Thursday because of the growing Syria crisis.

Ban has already spoken this week to US President Barack Obama and Secretary of State John Kerry about the worsening tensions.

"Starting tomorrow he will try to reach out to member states and take discussions forward on the question of what is happening in Syria," the spokesman said.

Ban has appealed for the inspectors to be allowed to complete their work before the major powers decide any follow-up action.

The United States, Britain and France are pressing for a military strike against President Bashar al-Assad, blaming him for the attack near Damascus on August 21 in which hundreds of people died.

The UN team have been in Syria since August 18 but have not yet given a formal verdict about whether banned chemical arms were used.

The inspectors "will have a large number of facts at their disposal," said Haq.

"They have collected a considerable amount of evidence -- evidence through samples, evidence through witness interviews -- and they can construct through that a fact-based narrative that can get at the key facts of what happened on August 21," he added.

The inspectors will give Ban an "oral briefing" as soon as they leave Syria, Haq said.

Some will also accompany the samples of blood, urine and hair as they are taken to laboratories across Europe.

"Some of the inspectors will be on hand for all the various deliveries to the various laboratory sites to ensure the chain of custody of the evidence they have collected, and they will wait for the laboratory analyses to be completed," he said.

"It's something that would take longer than days," the spokesman added.

"It is imperative that the work that the investigation team does be seen by all as fair, impartial and accurate. And so they will do their very best for accuracy while trying to get all the results in as soon as they can," Haq said.