Around-the-clock train and truck convoys are moving U.S. military hardware from the tense border with North Korea (search) as the American Army prepares to redeploy 3,600 troops to Iraq.

The massive logistical feat began July 7 and is moving hundreds of Abrams tanks, Bradley fighting vehicles (search), Humvees and artillery pieces to the southern port city of Busan to be shipped out under tight security.

About 3,600 troops from the U.S. Army's 2nd Infantry Division (search), dug into encampments between Seoul and the heavily fortified border with North Korea, will follow their equipment to Iraq later this summer.

The redeployment — one of the biggest realignments in a decade along the Cold War's last frontier — was announced in May and signals the first significant change of U.S. troop levels in South Korea since the early 1990s.

It underscores how the U.S. military is stretched to provide enough forces to cope with spiraling violence in Iraq while also meeting its other commitments.

U.S. Army spokeswoman Maj. Kathleen Johnson said Friday the transfer of military hardware to Busan is ongoing. But citing security reasons, she could not say when the transfer would be complete, when the equipment would be shipped to Iraq, or how many ships would be involved.

The equipment is arriving around-the-clock at the Busan port, where soldiers are working two 12-hour shifts, she said.

"This is a very intensive operation, involving a large amount of equipment," Johnson said. "The scale of this operation is about five times that of what we ordinarily do."

The 2nd Infantry Division's 14,000 troops form the mainstay of the 37,000 U.S. troops in South Korea.

For decades, the U.S. troops have buttressed South Korea's 650,000-member military in guarding against the communist North's 1.1 million-member military, the world's fifth-largest.

Since announcing the redeployment of the 3,600 soldiers to Iraq, Washington also has said it plans to withdraw about a third of the remaining troops by the end of 2005 as part of a global realignment to make U.S. forces more responsive.

Talk of reducing the U.S. military presence is sometimes unsettling in South Korea, which still has painful memories of the North Korean invasion that triggered the 1950-53 Korean War.

Many South Koreans fear that a reduction in U.S. troops might shake the delicate military balance on the peninsula amid tensions over the North's nuclear weapons program. It also has prompted President Roh Moo-hyun to say his country is ready to play a bigger role in defending itself.

U.S. troops led U.N. forces during the Korean War, defending South Korea from a North Korea backed by China and the former Soviet Union. The U.S. troops have since stayed on.