WASHINGTON (Reuters) - Secretary of State Hillary Clinton said on Tuesday that an argument could be made that Syrian leader Bashar al-Assad is a war criminal.

"There would be an argument to be made that he would fit into that category," Clinton told a Senate committee hearing, responding to a question from Senator Lindsey Graham as to whether Assad could be called a war criminal.

But she added that using such labels "limits options to persuade leaders to step down from power."

Clinton was also asked whether she thought Assad was on his way out eventually. "I do," she said. "I just don't know how to define eventually."

Last week Clinton attended a "Friends of Syria" conference of Western and Arab powers in Tunis that urged Assad to stop the killing of civilians in his crackdown on a nearly year-old uprising against his rule.

But the outside world has proved powerless to halt the killing in Syria, where repression of initially peaceful protests has spawned an armed insurrection.

A U.N. panel has drawn up a list of Syrian officials who could be investigated for crimes against humanity as part of Assad's brutal crackdown on peaceful protests.

(Reporting By Susan Cornwell; Editing by Jackie Frank)