Mr Fossett has not been heard from since Monday evening

US Civil Air Patrol spokeswoman Major Cynthia Ryan has warned that finding Mr Fossett could take weeks not days.

Maj Ryan said rescue teams were "still scratching the surface".

Rescuers are using thermal imaging so they can scour day and night the remote mountain terrain where Mr Fossett's plane went missing on Monday.

Search planes took off again shortly after 0700 local time (1400 GMT) on Friday, the fifth day of the search.

There has been no word from Mr Fossett since the 63-year-old was seen taking off from a private airfield near Yerington, Nevada, about 80 miles (130km) south-east of Reno.

"As you can imagine, trying to make that needle stand out in a haystack that big is going to be a real challenge," Maj Ryan said.

"It's going to be frustrating for a lot of people who were hoping for results early on," she added, saying that typically searches of this nature "can go on for as long as two weeks and longer".

If the worst comes to the worst... Steve's lived his life to the full, and he hasn't wasted a minute of his life Sir Richard Branson In pictures: Fossett's records Profile: Steve Fossett Rescuers scour desert

Emergency workers are also using sonar to search for possible wreckage in Walker Lake, an 18 mile-long (28km) body of water south-east of the ranch where Mr Fossett took off from.

Nevada state police spokesman Chuck Allen was keen to emphasise that the authorities had not received any information indicating that Mr Fossett had crashed into the lake.

However, he said that a boat with sonar equipment that could detect large and fixed objects beneath the surface of the water was being used, "if only to rule it out".

Treacherous winds

Mr Fossett's family reported him missing after he failed to return from a trip which should have lasted just a few hours.

Adam Mayberry, a local pilot and former spokesman for the Reno-Tahoe International Airport, suggested Mr Fossett's single-engine Citabria may have been hit by treacherous winds.

SOME OF FOSSETT'S RECORDS 1998/2002: Long-distance for solo ballooning 2001/2002: Duration for solo ballooning 2002: First solo round-the-world balloon flight First balloon crossings of Asia, Africa, Europe, South America, South Atlantic, South Pacific, Indian Oceans Seven fastest speed sailing titles 13 World Sailing Speed Record Council titles 2001: Fastest transatlantic sailing 2004: Fastest round-the-world sailing Round-the-world titles for medium airplanes US transcontinental titles for non-military aircraft

"There's been times when I've been flying in the wind and my blood turns cold," Mr Mayberry said.

British billionaire Sir Richard Branson, who has partnered Mr Fossett on earlier record-breaking attempts, said the lack of news was worrying.

"He's the number one gliding pilot in the world, as well as the number one aviator in the world... If anybody could have glided [this plane] down, it would have been him," he said.

But he added: "If the worst comes to the worst... Steve's lived his life to the full, and he hasn't wasted a minute of his life."

Mr Fossett did not file a flight plan with the aviation authorities before taking off because he was not required to do so.

The only clue to his whereabouts is that he was intending to fly south.

It is also understood Mr Fossett had some communication equipment with him, including a special emergency watch which he could have used to signal his position.

Record-breaker

Mr Fossett made his fortune in the American financial services industry.

But he is best known for the impressive number of world records he has broken as a pilot, balloonist and sailor.

He has set 116 records in five different sports, more than 60 of which remain unbroken.

Last year Mr Fossett broke the world record for flying further than anyone else in history.

In March 2005, he became the first person to fly a plane solo around the world without refuelling.

And in 2002 he was the first person to fly a balloon around the world solo.