The US defence secretary, Leon Panetta, has pushed back against fresh demands for US military involvement in Syria to end President Bashar al-Assad's deadly crackdown on his people.

"What doesn't make sense is to take unilateral action right now," Panetta told the Senate Armed Services Committee Wednesday about advising President Barack Obama to dispatch US forces. "I've got to make very sure we know what the mission is … achieving that mission at what price."

The panel's top Republican, Senator John McCain, said the estimated 7,500 dead and the bloodshed calls for US leadership that a Democratic president, Bill Clinton, displayed during the Bosnian war in the 1990s and that Obama eventually showed on Libya last year.

"In past situations, America has led. We're not leading, Mr Secretary," McCain told Panetta.