The increase was due to overlapping troop rotations, said Lawrence DiRita, the chief Pentagon spokesman.

The previous high in US force levels was reached in January, when the number of US troops in the country rose to 159,000 during national elections.

"The last number I saw was 161,000, but you're going to start to see that come down pretty dramatically because that was in-place relief and holdovers," said DiRita.

Lieutenant General John Vines, the number two commander in Iraq, said in September that the numbers would rise for the October 15 constitutional referendum by only some 2,000 troops from a base level of 138,000.

He said at the time that the growth in the number of trained Iraqi security forces meant there was less need for a larger US troop buildup for the referendum, or for the upcoming December 15 national elections.

In the past the US military has built up force levels during key political milestones in anticipation of rising insurgent violence.

"For the next election, I wouldn't be surprised to see it go right back up to 160,000 based on puts and takes and in-place rotations and relief, and everything else," DiRita said.