The United States will ramp up its troop levels in Afghanistan over the next few months as some troops slated for Iraq are deployed instead to the troubled Asian country, President George W. Bush said Tuesday.

Bush announced during a speech at the National Defence University in Washington, D.C., that he will order about 8,000 of the 146,000 U.S. troops in Iraq to come home by February.

He credited coalition troops for progress in Iraq, allowing a drawdown of forces from that country so the military can focus on a "quiet surge" in Afghanistan where "huge challenges remain."

"This is a vast country. And unlike Iraq, it has few natural resources and has an underdeveloped infrastructure. Its democratic institutions are fragile. Its enemies are some of most hardened terrorists and extremists in world," said Bush.

The success of Afghanistan, he said, is critical to security in the U.S. and its partners.

"So today, I am announcing additional American troop deployments to Afghanistan," said Bush.

A marine battalion scheduled to go to Iraq in November will instead head to Afghanistan, followed in January by an additional army combat brigade, said Bush. The two combined are estimated to equal roughly 4,500 troops.

Bush also vowed to focus efforts on doubling the size of the Afghan national army over the next five years and increasing the involvement of the Afghan tribes.

Further withdrawals up to successor

Bush's announcement on bringing home troops from Iraq in February leaves U.S. troop strength in the country largely unchanged until the next president takes over.

He said that any decision on withdrawing additional troops in 2009 will be left to his successor after Bush leaves office in January.

"Here is the bottom line: while the enemy in Iraq is still dangerous, we have seized the offensive; Iraqi forces are becoming increasingly capable of leading and winning the fight," Bush said.

But Democrats quickly shot back that Bush isn't doing enough to get troops out of Iraq and into Afghanistan where violence is rising.

"The president's plan to reduce force levels in Iraq may seem to signal movement in the right direction, but it really defers troop reductions until the next administration," said the chairman of the House Armed Services Committee, Rep. Ike Skelton, D-Mo.

"More significant troop reductions in Iraq are needed so that we can start to rebuild U.S. military readiness and provide the additional forces needed to finish the fight in Afghanistan."

Democratic presidential nominee Barack Obama has advocated pulling all U.S. combat forces out of Iraq within 16 months of taking office.

Republican nominee John McCain has said he would rely on the advice of U.S. military commanders to determine the timing and pace of troop reductions.

Both candidates have said more troops are needed in Afghanistan, where there has been a resurgence of the Taliban and a growth in violence.

With files from the Associated Press