But several officials declined to say whether there has been any direct communications with the insurgents, nor whether any demands had been made for the soldier's return. The military was largely silent about details surrounding the kidnapping, believed the first such abduction of a U.S. service member in the nearly eight-year-old war.
"We are not providing further details to protect the soldier's well-being," said Capt. Elizabeth Mathias, a spokeswoman in Afghanistan. Officials would not release his name, rank or other details.
"We understand him to have been captured by militant forces," Mathias said. "We have all available resources out there looking for him and hopefully providing for his safe return."
U.S. troops were brought in from nearby areas to help with the search, which included helicopters, Afghan Army support and increased use of intelligence gathering resources, officials said.
There was no immediate public claim of responsibility from any insurgent group. But large sweeps of eastern Afghanistan along Pakistan's border are believed controlled by the Taliban faction known as the Haqqani network, which also operates on the Pakistan side in that nation's lawless tribal region.
The network is led by warlord Sirajuddin Haqqani, whom the U.S. has accused of masterminding beheadings and suicide bombings including the July 2008 attack on the Indian Embassy in Kabul that killed some 60 people. The Haqqani group was also linked to an assassination attempt earlier last year on Afghan president Hamid Karzai.
News of the suspected capture 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.
Afghan Police Gen. Nabi Mullakheil said the soldier went missing in eastern Paktika province, where there is an American base operating near the Pakistan border.
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.
Though he was missing, his body armor and weapon were found on the base, two officials said.
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, one of the officials said.
Two U.S. defense sources said the soldier "just walked off" post with three Afghans after he finished working. They said they had no explanation for why he left the base.
The missing man is an enlisted soldier serving in an Army infantry unit, and his family has been notified he is missing.
The unit was assigned to a combat outpost, one of a number of smaller bases set up by foreign forces in Afghanistan, the officials said.
Myriad insurgent groups operate in eastern Afghanistan, and the Taliban is only one of them. Zabiullah Mujaheed, a spokesman for the Taliban, could not confirm that the soldier was with any of their forces.
If the fears of the soldier's capture are borne out, insurgents could exploit the development by making demands such as a prisoner exchange or use it for propaganda.
A number of civilians have been abducted in Afghanistan including aid workers and journalists - both foreigners and Afghans.
But the only other service member that officials could recall who had been captured was Petty Officer 1st class Neil C. Roberts, a 32-year-old Navy SEAL who was captured during a battle.
Roberts fell from a Chinook and was captured and killed by al-Qaida just months after the start of the war, in March 2002. Later, a second helicopter returned under fire and dropped troops near where Roberts fell. Six more Americans died in the fighting.
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