WASHINGTON (AFP) - United States (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".

Mr 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 US, but around the world, have to recognise 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".