The US has welcomed a decision by the UK to redeploy troops to central Iraq after it asked for assistance.
Black Watch expect to be home from Iraq for Christmas
Five hundred Black Watch soldiers and 350 support personnel will move from Basra to the US sector in central Iraq, but remain under British control.
US State Department spokesman Richard Boucher praised Britain's support, saying it demonstrated "the kind of role that Britain is prepared to play".
Defence chief Gen Michael Walker said the troops would be home for Christmas.
The redeployment was announced by Defence Secretary Geoff Hoon on Thursday after ministers agreed to the US request, based on a recommendation from Gen Walker.
Mr Boucher said: "It just demonstrates, once again, the kind of role that Britain is prepared to play in a matter that affects their security and our security, the security of all of us, and that is stabilizing Iraq and helping the people of Iraq take control of their destiny and reconstruct their country."
Details of when and where Black Watch will go have not been disclosed, but officials have said it will be within a large area to the south and west of Baghdad.
Gen Walker said the troops will be under "tactical control" of the US commander, but "he has no authority to give them orders that would, in any way, be against the doctrine and training that we would undertake".
"This is an unusual deployment in the context of what has happened so far in Iraq, but this is business as usual for us in the military."
Gen Walker added: "We have a clear-cut task, within a clear-cut geography and time limits."
He reiterated the government's promise that Black Watch will be home for Christmas.
Mr Hoon told MPs there was a "compelling military operational justification" for the move, and it entailed a "militarily acceptable" level of risk.
Preparations for the redeployment have been under way at the Black Watch's base in Basra, in southern Iraq, for several days.
The regiment's Warrior armoured fighting vehicles have been reinforced to provide better protection against possible insurgent attacks.
Shadow Defence Secretary Nicholas Soames said the Tories supported the redeployment as "a necessary military contribution" but criticised the length of time taken to announce a decision, after initial reports of the US request emerged last Friday.