A veteran Republican congressman opposed to US missile strikes defended his call Wednesday for a Syrian war crimes tribunal, saying the world should hold that nation's chemical weapons users to account.

Representative Chris Smith said President Barack Obama should immediately press for the UN Security Council's establishment of a tribunal to try Syrian President Bashar al-Assad and members of his regime, as well as rebels who the lawmaker said are committing "atrocities" against their own people.

"If we made this a real priority, I think we could hold Assad and his abusers to account, as well as those on the rebel side," Smith told AFP.

Smith introduced his measure in the House of Representatives this week, and as of Wednesday it has about a dozen bipartisan cosponsors and supporters.

The resolution could gain steam amid widespread skepticism over possible US military intervention in Syria, where Assad stands accused of unleashing a poison gas attack last month that killed more than 1,400 people.

Or it could fizzle as lawmakers seek to back the president in his pursuit of a diplomatic solution put forward by Russia that would secure the Syrian regime's chemical weapons stash.

House leadership has not publicly taken a position on the measure, but Smith said it would be a powerful international step short of a military strike.

"A war crimes tribunal would say to Assad, 'You're going to do time. It's not a matter of if, it's a matter of when,'" Smith told congressional broadcaster C-Span.

He stressed the tribunal should be applied to both sides of the conflict.

"The rebels without a doubt, particularly the Al-Qaeda component of it, are committing atrocities" including against people "because they are Christians."

Smith noted that the International Criminal Court has been trying accused war criminals. But neither the United States nor Syria are party to the ICC, and its track record for convictions is marginal, he said.

House Republican Scott Rigell backs the tribunal measure as "an appropriate non-lethal, diplomatic next step" in the global response to Assad.

The Syrian strongman, Rigell said, "should spend every day for the rest of his life paying for the crimes he has committed against humanity."