Failure by the United States to take military action against Syria would send a dangerous signal to Iran, Hezbollah and other US foes, Secretary of State John Kerry said Tuesday.

"Iran is hoping you look the other way," Kerry told lawmakers as he made the case for Congress to endorse punitive US strikes against the Syrian regime over its suspected use of chemical weapons.

"Our inaction would surely give them (Iran) a permission slip for them to at least misinterpret our intention, if not to put it to the test," Kerry told the Senate Foreign Relations Committee.

Hezbollah militants in Lebanon are "hoping that isolationism will prevail" and "North Korea is hoping that ambivalence carries the day," he said.

"They are all listening for our silence."