WASHINGTON, Aug 16 (Reuters) - The number of U.S. troops in Iraq will reach a new high of more than 170,000 later this year as departing and incoming units overlap for a period, a senior U.S. officer said on Thursday.

Army Lt. Gen. Carter Ham said the overlap would occur under plans to maintain the current level of around 160,000 troops into next spring but those plans could be changed if a political decision was made to draw down forces in Iraq.

"This fall ... there will a period where there will be up to five brigades simultaneously transitioning and so we'll see a spike during that transition period up to as high as perhaps 171,000," he told reporters at the Pentagon.

"Then as the transition completes we'll get back to the levels that we are (at) today," said Ham, director of operations for the U.S. military's Joint Staff.

A surge of some 30,000 extra troops has deployed to Iraq this year under a plan by President George W. Bush to crack down on sectarian violence and insurgent attacks.

Democrats, who control Congress, opposed the buildup and want Bush to begin pulling out troops soon. Bush has said his plan must be given time to work and any troop reductions should be decided in consultation with military commanders.

The two sides are set to clash next month, when Army Gen. David Petraeus, the top U.S. commander in Iraq, and Ryan Crocker, the U.S. ambassador in Baghdad, present a progress report.

The current level of 162,000 U.S. troops in Iraq is itself a record high for the war that began with the 2003 invasion by U.S.-led forces to topple Saddam Hussein, according to Pentagon figures.

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