The Pentagon plans to cut troop numbers in Iraq to 50,000 by September 1, from 176,000 at the peak of the deployment after the 2003 invasion to topple president Saddam Hussein, and end its official combat mission there.

"More than 140,000 troops were in Iraq on inauguration day (January 20, 2009)," Biden said in a speech to U.S. military veterans in Indiana. "By the end of August, 50,000 will remain."

Iraqi insurgents have been seeking to exploit a political vacuum created by the failure of the country's Sunni, Shi'ite and Kurdish factions to agree on a coalition government almost after an inconclusive March 7 parliamentary election seen as a crucial test for Iraq's young democracy.

Amid fears a weak Iraqi government could not preclude a lapse back into dictatorship, Biden said he expected they would form a strong one.

U.S. troops will stay in Iraq in an "advise-train-assist" role until the end of 2011.