WASHINGTON — Syrian leader Bashar al-Assad cannot sustain his hold on power and it is a matter of time before his leadership falls, U.S. Director of National Intelligence James Clapper said on Tuesday.

Syrian government forces reasserted control of Damascus suburbs on Tuesday after beating back rebels, and diplomatic pressure mounted on Assad as Western and Arab diplomats pushed for a U.N. Security Council resolution which would call for the Syrian leader to step down.

"I personally believe it's a question of time before Assad falls, but that's the issue, it could be a long time," Clapper said at a Senate intelligence committee hearing.

"Protraction of these demonstrations, the opposition continues to be fragmented, but I do not see how he can sustain his rule of Syria," he told U.S. lawmakers.

The unrest in Syria is of great concern to Iran and Hezbollah, which is "why they are both expending great effort in terms of resources and advice ... in trying to prop up the Assad regime," Clapper said.