US President Barack Obama on Friday promised a "vigorous investigation" into reports Syrian forces fired chemical weapons and renewed his warning that proof of their use would be a "game changer."

Obama delivered the warning during talks at the White House with King Abdullah II of Jordan, as he faced rising political pressure for a military intervention in the vicious Syrian civil war.

He told reporters that US authorities had "some evidence that chemical weapons have been used on the population in Syria, these are preliminary assessments, they're based on our intelligence gathering."

"We have varying degrees of confidence about the actual use, there's a range of questions about how, when, where these weapons have been used," he said.

The president said that Washington would pursue a "very vigourous investigation and would work with its partners towards a definitive answer on the chemical weapons issues as soon as possible.

He said that as horrific as it was that civilians face mortar fire and other attacks, the use of chemical weapons "crosses another line."

"That is going to be a game changer ... we have to make assessments deliberately but I think all of us, not just the United States, but around the world, have to recognize how we cannot stand by and permit the systematic of weapons like chemical weapons on civilian populations.

"This is going to be something that we're going to be paying a lot of attention to, trying to confirm, mobilize the international community around those issues"

Investigators collecting Syria chemical weapons evidence: UN
United Nations (AFP) April 26, 2013 - UN investigators have started collecting evidence outside Syria on the suspected use of chemical weapons while UN leader Ban Ki-moon presses for access, a spokesman said Friday.

Ban wrote a new letter to President Bashar al-Assad on Thursday seeking access as the United States revealed its suspicions that chemical arms have been used, UN spokesman Martin Nesirky told reporters.

"The secretary general urges the Syrian government to respond swiftly and favorably so that this mission can carry out its work in Syria," Nesirky said.

Ban named Swedish expert Ake Sellstrom on March 21 to head the UN inquiry after a request by Syria, Britain and France.

Sellstrom, a veteran of international weapons investigations in Iraq, will be in New York on Monday for talks with the UN leader on the efforts carried out so far.

He has already been in London this week and some information was handed over, diplomats said.

The Assad government has refused to let the investigators into the country, demanding that the inquiry be limited to its claims that chemical weapons were used near Aleppo on March 19.

Britain and France want it expanded to include opposition claims that chemical arms had also been used in Homs and near Damascus.

The UN team has been in Cyprus waiting but Nesirky said "they have been doing quite a lot, precisely because information has been provided and information is available without actually visiting Syria."

The experts "have been collating and analyzing the evidence and the information that is available to date from outside."

Western nations are believed to have handed over to the UN the accounts of witnesses who have fled Syria and some samples, diplomats said.

Nesirky said that while waiting for access the Sellstrom team "will continue with these offsite activities and that may include visits to relevant capitals."

The US administration said Thursday that Syria probably used chemical weapons on a "small scale," during the two-year-old conflict. Britain's Prime Minister David Cameron said Friday there was growing evidence of their use.