United States special forces scouring southern Afghanistan for Osama bin Laden have started using remote sensors capable of homing in on heat and vibrations through 100 feet of solid rock.
British troops have now joined the hunt for the cave complex where the Saudi dissident suspected of being behind the September 11 attacks on America is believed to be hiding out, the Foreign Secretary, Jack Straw, disclosed yesterday.
It is hoped the use of the sophisticated American technology will give the allied forces the element of surprise. Some of the devices are airborne, with the C-130 Hercules reconnaissance planes and unmanned spy aircraft equipped with thermal imaging cameras able to "see" objects 30 miles away. But others are small enough and light enough to be mounted on rifles, allowing marksmen to spot a human figure over a mile away.
Allied troops hope the technology will be particularly useful now that members of Mr Bin Laden's al-Qa'ida network have stopped using mobile phones, which can be intercepted by spy planes. The heat-sensing devices will become even more effective with the onset of the harsh Afghan winter as the contrast between the warm caves and the outside atmosphere grows more distinct.
"As it gets colder the caves are going to stay warm. Openings that release that air are going to be seen as a hot spot" a Government scientist told The Independent.
Some of the devices now being deployed are so sensitive they can identify the breath of a soldier or the pollution from the exhaust of a tank. Darkness or bad weather are no obstacles: a parked car can be detected in total darkness.
Metal equipment stored in underground caves give off weak magnetic fields which can be detected even up to 100 feet above ground. Even the faint magnetic fields emitted by wire cables, used to bring lighting into the tunnel mazes used by al-Qa'ida, can be detected by the high-tech scanners the American forces now possess, according to the New York Times.
The US Navy, meanwhile, is preparing to stop and board all freighters in waters off Pakistan and Iran in case they are carrying members of the al-Qa'ida network fleeing Afghanistan. The mission will be carried out mainly by ships currently escorting the 5th Fleet of US aircraft carriers and warships to the region. As the amount of territory under Taliban control in Afghanistan shrinks, the US does not want anyone, particularly Mr bin Laden, slipping through the net.
The order to begin board-and-search operations at sea had been given, "as a precautionary measure," said Marine Corps General Peter Pace, vice-chairman of the Joint Chiefs of Staff. "We are looking at how they might try to flee the country".
The US Navy has teams experienced in such boarding operations. Many have previously worked in the waters of the Persian Gulf, stopping and searching ships suspected of smuggling, in violation of international sanctions, for Iraq.
Retired Admiral Stan Arthur, who commanded the naval vessels in the Persian Gulf during the Gulf War, warned of the dangers of forcing a ship to heave to on the high seas. "The team is most vulnerable just as they're trying to board the vessel," he said.
Boarding a ship that did not want to be boarded was a hazardous exercise, with dark corridors, cramped and unfamiliar areas below deck and the prospect of hostile fire. "It's hard work on the young folks that do that," Arthur said. "There is a lot of climbing and crawling around."
Officials in Washington said they had not yet received specific information about Mr bin Laden or any of his cohorts making for the ocean. There would be wide choice of options for such a plan, with Pakistan and Iran offering plenty of remote coastal areas.
Donald Rumsfeld, the US Secretary of Defence, let slip that he would prefer to see Mr bin Laden delivered to him dead rather than alive.
Asked about the US mission to capture him "dead or alive", Mr Rumsfeld said: "Well, I don't know if it's politically correct to say you'd prefer the former, but I guess I'd prefer the former myself".
A stop-and-search operation involving a US ship, the USS Peterson, went awry in the Gulf last week. Two US sailors were lost feared drowned when they boarded a rusting tanker suspecting of smuggling oil for Iraq. It sank while they were on board.