WASHINGTON, Jan 1: The Pentagon is gearing up for a massive rotation of about a quarter million troops in and out of Iraq, a giant logistics chore complicated by concerns about opportunistic attacks targeting Americans as they arrive or depart.

Between late January and May, 123,000 weary US troops will be pulled out of Iraq and replaced with about 110,000 fresh Army soldiers and Marines. In addition, 11,000 US troops in Afghanistan will be brought home and replaced with about the same number.

Air Force Gen. Richard Myers, chairman of the military's Joint Chiefs of Staff, called the rotation "a logistics feat that will rival any in history". "So there's going to be a lot of turbulence in the system, as you would expect," Gen Myers told reporters recently.

"Turbulence is always undesirable," added Defence Secretary Donald Rumsfeld. A total of 212 US soldiers have been killed in combat since US President George Bush declared an end to major combat operations in Iraq on May 1. Attacks by guerillas regularly add to the death toll.

Pentagon officials and defence analysts said the guerillas may view the mass arrivals and departures as a unique opportunity to inflict further casualties on Americans.

"The troops will be on the roads, they will be in the skies, and, in general, they will be away from fortified areas in larger numbers than in any time since combat ended," said analyst Loren Thompson of the Lexington Institute.

Analyst Anthony Cordesman of the Center for Strategic and International Studies added: "There's always the risk that some terrorist group or insurgent group can hit a US aircraft either taking off or landing, and this is particularly true during periods of high density when it may not be possible to stagger the aircraft quite as securely as usual."

A Pentagon official also noted the danger of a spate of deadly vehicle accidents with such a large movement of forces. Defence officials expressed concern about the wholesale withdrawal of the US forces who have carried the load in Iraq and their replacement with troops and commanders lacking experience in Iraq.

"The turnover of people - you lose situational awareness, you lose relationships, you lose the experience," Mr Rumsfeld said. "The people going over are ready, but the people there are experienced and really know their stuff. And who would you rather have there?"

"So what we're going to have to do is to manage that transition very carefully. There's going to have to be overlap," he said. Officials said each arriving unit is due to have a period of several weeks to work alongside the departing unit to get briefed on terrain, culture and the nature of the enemy in their particular region.

The rotation of people will be accompanied by a change in equipment to better fight a guerilla war rather than organized military units. There will be fewer heavy M-1A1 Abrams tanks and Bradley Fighting Vehicles, but more lighter armoured Humvees intended to protect against rocket-propelled grenades or roadside bombs used by insurgents, officials said.-Reuters