The secret military study reportedly says that a sudden pull-out by the US military could result in all out civil war.

The group conducting the review is likely to recommend a small, short-term increase in troops in conjunction with a long-term commitment to increased training and advising of Iraqi forces, officials told the Washington Post.

Under this combined option, the US presence in Iraq, currently about 140,000 troops, would be boosted by 20,000 to 30,000 for a short time, officials said.

The secret military study was commissioned by General Peter Pace, who as chairman of the Joint Chiefs of Staff is the most senior uniformed officer in the US.

The White House is reconsidering its Iraq policy after a decisive defeat for the Republicans in congressional elections. The loss of the senate and House of Representatives has been largely blamed on voters' desire for an end to military casualties.

Advisory commission

A special advisory commission led by James Baker, a Bush family friend and former secretary of state, and former politician Lee Hamilton is also expected to issue its report soon.

"[The US should] ... let the Iraqi leadership know we're not going to be staying"

Joseph Biden, US Democrat senator

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There is speculation that its members would propose a way ahead for Iraq while making clear that the US military mission should not last indefinitely.

The commission is expected to release its findings and recommendations next month.

Joseph Biden, a Democrat senator who will take over the foreign relations committee in January, said he would like the commission to assert that US troop commitments are not open-ended, propose a clear political plan for Iraq; and recommend engaging Iraq's neighbours in a political and diplomatic solution.

The United States should "begin to let the Iraqi leadership know we're not going to be staying", he said in a television interview on Monday.