The "vast majority" of remaining US troops in Iraq will be out by mid-December as the American military's withdrawal picks up pace, a US general said Thursday.

Less than 34,000 troops remain in Iraq after reaching a peak of 170,000 in 2007 during a buildup ordered by former president George W. Bush, said Major General Thomas Spoehr, deputy commanding general for the US force in Iraq.

"As I look at the plan, I think it's clear to me that by the time we get to about mid-December or so, the vast majority of the US forces in Iraq -- we plan to have them withdrawn from Iraq by that time," he said via video link from Baghdad.

Under a 2008 security pact, the United States has to pull out all troops by the end of the year. Negotiations for a possibly smaller post-2011 force of a few thousand faltered over the question of legal immunity for American soldiers.

The withdrawal of troops and equipment represented an "immense" logistical effort, with about 1,650 trucks traveling up and down the country on any given day, Spoehr told reporters.

The general compared the undertaking to the famed "Red Ball Express" during World War II, when allied forces rushed supplies by truck to combat troops advancing toward Germany after the D-Day landing.

"I will tell you that right now, as we sit here, we are deep in the midst of this. So there are trucks and planes and people moving very quickly at a high rate of speed throughout Iraq to execute our commitments," Spoehr said.

But he added that the withdrawal would be carried out in a "measured way" to guard against potential insurgent attacks as the troops depart.

US officers have therefore decided not to divulge details of the planned withdrawal as information released previously about base closures appeared to have prompted attacks, according to Spoehr.

US forces, which once operated out of 505 bases during the height of the "surge" of additional troops in 2007, now have only 12 bases left in the country.

Most troops will be leaving by air with truck convoys carrying equipment to neighboring Kuwait.

President Barack Obama's administration is weighing a possible expansion of the US military's presence in Kuwait and the Gulf region after the troop withdrawal from Iraq, in a move officials say would be designed to counter the threat posed by Tehran.