Army Gen. David Petraeus, the top U.S. commander in Iraq, faced criticism from a Democratic presidential contender on Tuesday over America's war strategy.

Delaware Sen. Joseph Biden voiced his criticism at the start of a session of the U.S. Senate's foreign relations committee to hear testimony from Petraeus and the U.S. ambassador to Iraq, Ryan Crocker.

"The American people will not support an infinite war whose sole remaining purpose is to prevent the situation in Iraq from becoming worse than it is today," Biden said.

Petraeus outlined a plan to reduce the number of troops in Iraq by some 30,000 to around 130,000 by next summer, a return to the level before President George W. Bush ordered a surge of forces at the start of this year.

But Petraeus, testifying on Monday before committees of the House of Representatives, said he could not predict how quickly troop levels would fall beyond next summer and insisted his force should continue to protect the Iraqi population.

He rejected suggestions that his troops should switch to a mission focused on handing over duties to Iraqi forces and conducting counter-terrorism operations.

Biden said the surge strategy could not succeed as national Iraqi leaders were not committed to reconciliation between sectarian and ethnic groups.

"It's time to turn the corner," Biden said. "We should stop the surge and start bringing our troops home."

Iraq George W. Bush Joe biden