Los Angeles: The search for adventurer Steve Fossett was expanded by thousands of square miles as it entered a fourth day yesterday, with rescuers warning it could be weeks before the missing aviator is found.

Major Cynthia Ryan of the Civil Air Patrol told reporters that aircraft were now scouring a larger area of rugged mountain terrain in northwest Nevada, where Fossett was believed to be flying when his plane vanished on Monday. "The search area has expanded with the accumulated passes and that kind of thing, to an area in excess of 10,000 square miles (25,900 square kilometers)," Ryan said.

Previously rescuers had been scouring a 600-square-mile area. "As you can imagine, trying to make that needle stand out in a haystack that big is going to be a real challenge," Ryan added. Several planes took off in perfect conditions just after daybreak to resume hunting for Fossett while a C-130 with infra-red scanners had flown through the night looking for clues to his whereabouts.

A total of 10 aircraft were patrolling the skies later yesterday as the hunt intensified. Ryan emphasized that it was necessary for aircraft to make several passes over the same ground in order to definitively rule out an area of terrain. "Searches of this nature -- typically, they can go on for as long as two weeks and longer," Ryan said.

"Four days into it, we are still scratching the surface." Earlier, police said a search and rescue boat fitted with sonar equipment would be deployed on a lake near the Hilton Flying M Ranch, 80 miles (130 km) southeast of Reno, Nevada where Fossett was last seen.