Desperate search under way for US unit missing in action

Meanwhile, the new batch of rescuers were having to contend with the threat of rebel attacks, and the prospect that, according to a Taliban spokesman, militants had captured one of the original unit. The US military is now in the midst of what is proving the worst week since the Taliban were overthrown in 2001.

Elsewhere in Afghanistan, 18 rebels and two Afghan soldiers were killed in an assault on a Taliban hide-out in mountains where about 100 insurgents were thought to be camped. This operation comes after fighting in the region left 25 people dead, including nine tribal elders who Taliban rebels kidnapped and then killed, apparently in retaliation for the deaths of their own.

The loss of the American military team in the remote eastern mountains worsened the already stinging blow suffered by the military after 16 troops were killed on Tuesday aboard the MH-47 Chinook. This was single biggest loss for US forces in Afghanistan since they overthrew the Taliban in 2001. The remains of the troops - eight from airborne Special Forces units and eight Navy Seal commandos - were flown back to the US on Friday night.

The crisis over the missing unit comes as the Americans scramble to deal with an insurgency that threatens three years of progress toward peace. US forces were using "every available asset" to search for the missing men, said spokesman Lt Col Jerry O'Hara said. The troops are a small team from the special operations forces, said military officials. The BBC reported that a number of Afghan guides working with the U.S. military were also missing.

The downed helicopter had been trying to "extract the soldiers" on Tuesday when it went into the mountains near Asadabad, close to the Pakistani border, O'Hara said. "All our hopes are that we find our missing service members. On top of those hopes are actions on the ground looking for them," he said. "It's a very demanding area: very mountainous, very wooded and the likelihood of enemy contact is probable."

The Taliban claim to have captured one of the men came from purported spokesman Mullah Latif Hakimi. He said a "high-ranking American" was caught in the same area as where the helicopter went down. Lt Col O'Hara said there was "no proof or evidence indicating anything other than the soldiers are missing." On Friday the US military said it could neither confirm nor deny a claim by Taliban spokesman Hakimi that insurgents killed seven US "spies" before the Chinook was shot down.

On the same day, Hakimi said guerrillas in Kunar captured an American soldier on Wednesday who had been aboard the helicopter when it crashed. Hakimi, who also claimed insurgents shot down the helicopter, often calls news organisations to take responsibility for attacks, and the information frequently proves exaggerated or untrue. His exact tie to the Taliban leadership is unclear.

The loss of the helicopter, the missing men and the fierce clashes in central Afghanistan follow three months of unprecedented fighting that has killed about 495 suspected insurgents, 49 Afghan police and soldiers, 134 civilians, and 45 US troops. The American casualties came as insurgents have stepped up their activity to try to derail parliamentary elections on 18 September, the next big step in Afghanistan's difficult path to stability.

Only eight months ago, Afghan and US officials were hailing a relatively peaceful presidential election as a sign that the Taliban rebellion was finished. But remnants of the former Taliban regime have stepped up attacks, and there are disturbing signs that foreign fighters - including some linked to al-Qa'ida - might be making a new push to sow an Iraq-style insurgency. Afghan officials say the fighters have used the porous border with Pakistan to enter the country, and officials have called on the Pakistani government do more to stop them.