Steve Fossett (2006 photo)

Search and rescue operations for American adventurer Steve Fossett entered a fourth day Thursday in the western U.S. state of Nevada.

Aerial operations for the 63-year-old millionaire continued early this morning in conditions authorities describe as excellent.

Officers of Nevada's Civil Air Patrol and National Guard said today airplanes and helicopters outfitted with special equipment have been searching for Fossett almost 24 hours a day since he was reported missing late Monday.

They say the search area in the mountainous, high desert region has been expanded to 26,000 square kilometers.

Fossett vanished from radar Monday after taking off in a small, single engine airplane on a flight that was supposed to have lasted only a few hours.

Although no signal from his emergency equipment has been detected, rescuers said today they are optimistic they will find him.

Fossett was intending to fly over dry lake beds, scouting for a location to stage an attempt at the world's land speed record.

The wealthy daredevil, who swam the English Channel, is well-known for his record-setting adventures in balloons, sailboats, gliders and powered aircraft.

In 2002, Fossett became the first person to fly solo around the world in a balloon. And in 2005, he set the record as the first person to fly an airplane solo around the world without refueling.