Syrian president Bashar al-Assad could be labelled a "war criminal", US Secretary of State Hillary Clinton has said.

"There would be an argument to be made that he would fit into that category," Clinton told a US Senate committee hearing.

She was asked by Senator Lindsey Graham if Assad could be classed a war criminal. She responded in diplomatic language. Such labels "limit options to persuade leaders to step down from power", she warned.

She said she believed Assad would leave power but added: "I just don't know how to define eventually."

Assad's forces have cracked down hard on an 11-month uprising that had left more than 5,400 people dead by the end of January, according to the UN.

Clinton has been stepping up pressure on Assad in the last few months and has called several times for him to step down.