KABUL (AP) - Insurgents have captured an American soldier in eastern Afghanistan after he walked off post with his three Afghan counterparts, officials said Thursday.

Spokeswoman Capt. Elizabeth Mathias said the soldier disappeared Tuesday.

"We have all available resources out there looking for him and hopefully providing for his safe return," Mathias said.

Mathias did not provide details on the soldier, the location where he was captured or the circumstances.

The news broke as thousands of U.S. Marines launched a major anti-Taliban offensive in southern Afghanistan. The missing soldier was not part of that operation.

"We are not providing further details to protect the soldier's well-being," she said.

Afghan Police Gen. Nabi Mullakheil said the soldier went missing in the Mullakheil area of eastern Paktika province, where there is an American base.

The soldier was noticed missing during a routine check of the unit on Tuesday and was first listed as "duty status whereabouts unknown," a U.S. defense official said on condition of anonymity.

It wasn't until Thursday that officials said publicly that he was missing and described him as "believed captured." Details of such incidents are routinely held very tightly by the military as it works to retrieve a missing or captured soldier without giving away any information to captors.

Initial reports indicated that the soldier was off duty at the time he went missing, having just completed a shift, the official said on condition of anonymity because details are still sketchy.

The missing man is an enlisted soldier, and his family has been notified.

Two U.S. defense sources said the soldier "just walked off" post with three Afghan counterparts after he finished working. They said they had no explanation for why he left the base. He was assigned to a combat outpost, one of a number of smaller bases set up by foreign forces in Afghanistan, the officials said.

Zabiullah Mujaheed, a spokesman for the Taliban, could not confirm that the soldier was with any of their forces. A myriad of insurgent groups operate in eastern Afghanistan, and the Taliban is only one of them.

The most important insurgent group operating in that area is known as Haqqani network and is led by Siraj Haqqani, whom the U.S. has accused of masterminding beheadings and suicide bombings.

Associated Press writer Pauline Jelinek in Washington contributed to this report.

(Copyright 2009 by The Associated Press. All Rights Reserved.)