US Defence Secretary Leon Panetta says Syrian President Bashar al-Assad's assault on the city of Aleppo will be "a nail in his coffin".
Mr Panetta was speaking at the start of a five-day Middle East tour.
Heavy fighting is continuing in Syria's largest city where government forces are trying to oust rebel fighters.
UN humanitarian chief Valerie Amos says 200,000 people have fled the fighting in Aleppo and an unknown number are trapped.
She says the city urgently needs supplies including food and water.
Mr Panetta says the Syrian crisis is deepening and President Assad is hastening his own demise.