WASHINGTON (Reuters) - President George W. Bush will cut U.S. force levels in Iraq only modestly over the rest of his term, pulling 8,000 troops out by February, when his successor will have taken over as commander in chief.

Bush, an unpopular president managing an unpopular war, will say on Tuesday that a dramatic drop in violence in Iraq allows the Pentagon to bring troops home and shift more forces to Afghanistan, where attacks by Islamist militants have soared over two years.

"While the enemy in Iraq is still dangerous, we have seized the offensive, and Iraqi forces are becoming increasingly capable of leading and winning the fight," Bush plans to say at the National Defense University, according to his prepared remarks released on Monday by the White House.

"And if the progress in Iraq continues to hold, Gen. (David) Petraeus and our military leaders believe additional reductions will be possible in the first half of 2009."

A cut of 8,000 would leave 138,000 U.S. troops in Iraq -- still more than before Bush ordered a "surge" of extra forces in 2007 and also more than in November 2006, when his Republicans lost mid-term congressional elections largely due to voter anger over the war.

Bush's plan follows recommendations from top U.S. defense officials, including Defense Secretary Robert Gates, Chairman of the Joint Chiefs of Staff Adm. Mike Mullen and Petraeus, the top commander in Iraq.

But any large-scale shift in U.S. forces in Iraq and Afghanistan will be left to Bush's successor -- either Republican Sen. John McCain or Democratic Sen. Barack Obama. Bush will leave office in January 2009 after the Nov. 4 election.

Obama has promised to withdraw combat troops from Iraq within 16 months and said he would put more resources into Afghanistan and counterterrorism efforts along the Pakistan border, where U.S. officials say they believe al Qaeda leader Osama bin Laden is hiding.

McCain has steadfastly refused any set timeline for withdrawing U.S. forces from Iraq and instead prefers Bush's policy of removing them based on commanders' recommendations and security conditions in the war zone.

SHIFTING FOCUS

Bush, in his speech on Tuesday, will point to data showing violence in Iraq has dropped to levels not seen since 2004.

But he will caution that progress in Iraq, which U.S.-led forces invaded in March 2003 to oust Saddam Hussein, remains "fragile and reversible."

Bush's surge strategy, which added about 30,000 troops to Iraq, has been credited with helping to reduce violence and pulling Iraq back from the brink of civil war. The surge, when announced in 2007, was criticized by many Democrats who said the United States should be pulling out.

About 20,000 of those surge troops have returned home without replacement, leaving 146,000 U.S. troops in Iraq.

Bush will say some 3,400 support troops will leave Iraq over the coming months, along with a Marine battalion by November. One of the 15 Army combat brigades will leave in February, bringing the total to about 8,000 withdrawn, Bush will say.

But as violence has fallen in Iraq, attacks against U.S., NATO and Afghan troops in Afghanistan have soared. NATO commanders there have asked for additional forces for years and say they still need about 12,000 troops.

The United States has 33,000 troops in Afghanistan, split between a NATO-led mission and a separate counterterrorism mission run by the U.S. military.

Bush will say he plans to shift more troops to Afghanistan, which U.S.-led coalition forces invaded in October 2001 to remove the fundamentalist Taliban government in the wake of the Sept. 11 attacks on the United States by al Qaeda militants.

"In November, a Marine battalion that was scheduled to deploy to Iraq will instead deploy to Afghanistan," according to the speech text. "It will be followed in January by an Army combat brigade."