WASHINGTON, July 31 (Xinhua) -- The Pentagon said on Tuesday that it will send some 20,000 troops to Iraq to replace soldiers rotating out.

The soldiers are from the Army's 3rd Brigade of the 4th Infantry Division, the I Marine Expeditionary Force Headquarters, and two regimental combat teams, Pentagon spokesman Bryan Whitman told reporters.

Except for the Marine headquarters, all of the troops will have to begin arriving in Iraq in December, Whitman said.

He did not give a precise date for when the headquarters will deploy, except that it was "into the next year."

The troops are not part of the security "surge" that has brought the current number of troops in Iraq to 159,000, Whitman said.

Instead, "this is part of the normal planning for the normal rotation," he added.

Rather than talking about troop numbers, the Pentagon has been characterizing troop strength in Iraq by talking about the number of combat brigades deployed to the region. The typical Army combat brigade has 3,500 troops.

The security "surge" that began in January added an additional five combat brigades, for a total of 20 combat brigades.