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US President Barack Obama has vowed a "vigorous investigation" into reports Syria has used chemical weapons, warning they will be a "game changer" for US policy if proven true, BBC reported. 

Both the US and UK have pointed to emerging evidence that Syria has used weapons such as the nerve gas sarin.

UK Prime Minister David Cameron earlier told the BBC it appeared a war crime was being committed by Syria.

Syrian officials have denounced the allegations as "lies."

According to the UN, at least 70,000 people have been killed in the two-year uprising against Syrian President Bashar al-Assad.

Mr Obama was speaking as he met King Abdullah II of Jordan at the White House.

The US president said there was "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."

Mr Obama insisted more evidence was still needed and that there would be a "vigorous investigation."

But proof of their use would be a "game changer", he said.

"Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law.”