Four US soldiers have been killed in a mine blast southwest of Kabul in the southeastern Afghan province of Logar, the US military says.

"Four US troops were killed in action but we have no indication that this was a new mine or an IED (improvised explosive device)," US military spokeswoman Cindy Moore said on Saturday.

She said the bodies of the soldiers had already been evacuated from the area, 40km southwest of Kabul, and an investigation was ongoing to determine the cause of the explosion.

Afghanistan is littered with old, unexploded mines after a quarter century of war, and it was not immediately clear if Saturday's deaths were the result of an unexploded or freshly laid device.

Saturday's explosion brings to seven the number of US soldiers killed so far this year in Afghanistan.

A US soldier was killed in the western region Herat in a mine blast earlier this month.

PHOTO CAPTION

A visitor looks at the new exhibit at Arlington National Cemetery entitled: 'Faces of The Fallen,' featuring 1,300 portraits by 200 artists of US troops who died in the wars in Iraq and Afghanistan. (AFP/Paul)