The United States has begun a dramatic realignment of its military forces abroad, making key changes in the Middle East and Asia and preparing a restructuring in Europe to confront emerging 21st century threats.

US officials say the changes are intended to enable American forces to combat terrorism and rogue states armed with weapons of mass destruction.

Some moves already have been made in Saudi Arabia and South Korea in the aftermath of the Iraq war, and a major reduction in US forces in Germany is expected to follow.

A central element of the restructuring is the creation of stripped-down bases in eastern Europe, Central Asia and elsewhere, and a de-emphasis on big permanent bases in nations such as Germany, defense analysts said.

"We have been reviewing our presence around the world, in every portion of the globe," Defence Secretary Donald Rumsfeld said this week. He argues US forces still are arranged as if the Soviet Union existed.

Deputy Defence Secretary Paul Wolfowitz has said America now faces 'a very different threat' requiring a new approach. "It's appropriate to look at how those forces are postured, how we can get the most effectiveness out of them," he said.

"The concept behind the Pentagon's decision to change our basing structure is this ability to be flexible to respond to a broad array of unknown threats around the world," said defence analyst Charles Pena of the Cato Institute.

"So rather than lumping all your forces in one area that you think is strategic, you spread your forces around the world so that if something happens you have some ability to get there quickly."

Defence analyst Daniel Goure of the Lexington Institute said the realignment is the most significant since the retrenchment after the Cold War, when US troop levels in Europe plummeted from 300,000 to 100,000.

"In terms of moving into new places rather than coming back from places, this is the first time we have been doing that in just about 40 years," Goure said.

The United States announced in April that nearly all its 5,000 troops would be pulled out of Saudi Arabia, from which it had staged air patrols for a decade over southern Iraq. The move increased the importance of US military facilities in other states in the region such as Qatar, Kuwait and Bahrain.

On June 5, the United States and South Korea announced an agreement to shift US troops from the demilitarised zone, separating South Korea and North Korea, and establish hub bases south of Seoul.

Germany, an opponent of the U.S.-led war in Iraq, is expected to see a major decline in US troops, although Ramstein Air Base is expected to remain unscathed. Germany currently hosts 68,000 US troops, but US officials say many of them could be relocated, primarily to eastern Europe.

Pentagon officials have not revealed the scope of the reductions in Germany, a front-line state in the Cold War. Goure said the realignment may leave only 15,000 US troops there.

Former Soviet bloc countries such as Romania, Bulgaria and Poland could benefit from the shift of US forces out of Germany, with analysts noting it is cheaper to maintain bases in such places and there are fewer environmental and other constraints than in western Europe.

Patrick Garrett, an analyst with the Globalsecurity.org military think tank, said Turkey might end up like Saudi Arabia with virtually no US troop presence, particularly if America ends up with long-term access to bases in Iraq. He said the United States may give up Turkey's Incirlik Air Base.

Despite local resentment of US troops in Okinawa, Pentagon officials said there are no plans to relocate the 20,000 Marines stationed there to elsewhere in the Pacific.

Analysts said key Pacific countries for US forces in coming years might include Australia, Singapore and even Vietnam. A defence official said the Philippines, mentioned as a possible site for bases, has made it clear 'they have no desire' for that.