"Dirt got through the screen," a US intelligence official told New Yorker magazine.

According to the magazine, the US allowed Pakistan's military officials to be flown to safety to preserve the political standing of General Pervez Musharraf, whose survival is seen as crucial to the American war effort.

The exodus - which the administration insisted at the time had not taken place - was intended only to rescue Pakistani officials from Kunduz, which was surrounded by Northern Alliance troops backed by American forces.

But the New Yorker's defence correspondent, Seymour Hersh, quotes a senior intelligence source as saying that Taliban and al-Qaida fighters slipped on board. "Everyone brought their friends with them," a defence adviser told him.

The US military may even have directly co-operated in the airlifts, according to the article, which is based on conversations with intelligence officials and senior military officers. Two such sources told Hersh that the US central command was ordered to establish a special air corridor to guarantee that the rescue flights could proceed safely.

"Unhappy is not the word," said an analyst who worked with Delta Force, the commando unit charged with destroying Taliban bases on the ground, describing the army's reaction to the order.

Rumours of the airlifts came initially from Northern Alliance officials but were strenuously denied by Donald Rumsfeld, the US defence secretary. Asked about rescue aircraft, he said in November: "If we see them, we shoot them down." He said later: "Any idea that those people should be let loose on any basis at all to leave that country and destabilise other countries is unacceptable."

The Indian national security minister, Brajesh Mishra, is quoted as saying that as many as 5,000 Pakistanis were rescued. US sources suggested the total was much lower.