GlobalFlyer project manager Paul Moore said at Mission Control in Salina (Kansas) on Wednesday that the fuel situation may force the 60-year-old US millionaire to abort the flight, and a decision would be made when the aircraft approaches Hawaii in several hours.
Fossett, speaking by radio to reporters, called the previously undetected loss of 1181 kg of fuel in the first three and a half hours of the flight "a huge setback".
"I don't have a very high level of confidence at this point," Fossett said after flying over China. "I'm going to do the best I can."
At 1700 GMT, Fossett had left Japanese airspace and was heading east towards Hawaii at speeds of around 340 knots (633km per hour) and an altitude of some 13,000 metres.
Mercy of winds
Fossett said he would have to rely on tailwinds of at least 180km per hour, the average up to now, to finish his attempt.
The three-day journey will test
"Without that tailwind we won't be able to get back," Moore said. "Steve is quite literally at the mercy of the winds."
Organisers said the fuel loss may have been the result of inadequate venting of fuel tanks and greater-than-expected fuel burn. The Virgin Atlantic GlobalFlyer had never been tested at the full fuel conditions Fossett experienced on take-off.
"It's still not clear to us how or why the discrepancy has occurred or how or why the fuel disappeared," Moore said.
Earlier Wednesday, Fossett passed the halfway point in his effort to make the first solo, non-stop flight around the world without refuelling.
Fossett took off from Kansas on Monday and was to return on Thursday after spending between 60 and 80 hours in the air.
"It's still not clear to us how or why the discrepancy has occurred or how or why the fuel disappeared"
GlobalFlyer Project Manager
The 1522kg single-engine aircraft was to burn nearly 8636 kilos of fuel.
The three-day journey will test the endurance and piloting abilities of Fossett, who has set dozens of world records with jet airplanes and gliders, hot air balloons and sailing.
The GlobalFlyer was designed by aviation legend Burt Rutan, who recently made headlines when his SpaceShipOne won the $10 million X Prize for sending a privately designed craft into space twice in two weeks.