Gordon Brown has sought to head off growing speculation over an early British exit from Iraq, insisting troops still had "an important job to do".
In a letter to Liberal Democrat leader Sir Menzies Campbell, the Prime Minister rejected calls to set out a timetable for withdrawal. He said the military had "clear obligations to discharge", and it was wrong to say the continued presence of more than 5,000 personnel would "achieve little".
"Decisions on UK force levels and posture in Iraq are dictated by conditions on the ground," Mr Brown added.
Attacks on British forces in southern Iraq have been intensifying in recent months, with insurgents seeking to portray an expected withdrawal to an airbase outside Basra as an humiliating retreat.
Commentators have increasingly been voicing concerns about the UK's ability to sustain intense military campaigns in both Iraq and Afghanistan.
There have been repeated claims that UK forces are actually exacerbating the situation in southern Iraq, where sectarian factions are regarded as on the verge of civil war.
Mr Brown was responding to a letter from Sir Menzies in which he repeated demands for a reappraisal of Government strategy in Iraq and Afghanistan, insisting the level of casualties were now "unacceptable".
The message from the Premier is likely to be seen as an effort to reassure those who fear that Britain is wavering in its commitment to military operations - particularly among the US administration.
"It is wrong to say that the continuing presence of UK forces in Iraq will achieve little, or that they are severely restricted in what they can do," Mr Brown wrote. "UK forces in Basra continue to have the capability to strike against the militias and provide overall security.
"They will continue to work with the Iraqi authorities and security forces to get them to the point where they can assume full responsibility for security."
