US Secretary of State John Kerry said Friday there was "strong evidence" the regime of Syrian President Bashar al-Assad had used chemical weapons against rebel forces.

"This fight is about the terrible choices that the Assad regime has made with its willingness to kill anywhere... to use gas, which we believe there is strong evidence of use of," Kerry said during a Google+ hangout.

The US government cited for the first time two weeks ago the Syrian regime's possible use of chemical weapons against its own people, but President Barack Obama stressed there was insufficient proof to determine whether a "red line" had been crossed.

In an interview published Thursday, Vice President Joe Biden stressed that once the use of the chemical weapons has been verified, Obama would likely make a "proportional response in terms of meaningful action," without providing further details.

In other remarks Friday, Kerry turned to Syria's future.

"If you're willing to compromise in the choosing of the people who will run that transitional government, and you choose in good faith people who are prepared to put in front of the people of Syria a fair choice about who their leader ought to be, then I believe you could avoid war and you could have a settlement," he said.

He added, however, that the transitional government "will not include President Assad," and again referenced an international conference on Syria he announced in Moscow earlier this week for the end of May, confirming it could take place in Geneva.

"I can tell you that we all owe the world the best effort possible to try to get there and to explore, in good faith, whether or not we can end the violence, end the bloodshed, avoid a complete disintegration," Kerry said.

"And my judgment is that if we get to this meeting in Geneva, the arguments will be very clear to everybody as to who is prepared to be reasonable and who is not prepared to be reasonable."

Turkish Prime Minister Recep Tayyip Erdogan meanwhile told NBC News Thursday he believes Syria has used chemical weapons, crossing the "red line" set out by Obama.

The top US diplomat's remarks also came as British Prime Minister David Cameron and Russian President Vladimir Putin sought Friday to forge a joint approach to the Syrian crisis, as a row over Moscow's weapons deliveries to Damascus showed up the divisions between the Kremlin and the West.

The war in Syria has cost an estimated 70,000 lives and displaced millions of people, including hundreds of thousands who have fled to neighboring countries.