The space shuttle Columbia crashed in Texas yesterday with seven astronauts on board after it lost contact with NASA minutes before landing.

Television images showed several white trails in the blue sky, which could be an indication of the breakup of the shuttle because normally only a single trail is visible. Officials immediately warned about the possibility of toxic debris in the Dallas-Fort Worth area.

Columbia disappeared from NASA's radar screens at 9:00am (1400 GMT). It was due to land at 9:16am (1416 GMT).

There is no possibility that the US

Space Shuttle Columbia could have made a successful emergency landing from the high altitude and speed at which it was travelling when it lost contact with NASA, the US space agency said.

"I'm afraid that is not really an option at this altitude," NASA spokesman Kyle Herring told CNN.

The shuttle was flying at 200,000 feet at the time, moving at 20,000 kilometers (12,500 miles) per hour.

At that speed, it is not possible for the crewmembers to eject safely, NASA said.

A White House spokesman said President George W Bush had cut short his stay at the Camp David retreat and would monitor from the shuttle disappearance from the

White House.

"The president is returning to the White House so he can continue to monitor the situation from here," Scott McClellan said. "We are continuing to monitor the situation and awaiting more information from NASA."

At the Kennedy Space Centre, CNN showed flags flying at halfmast.

NASA dispatched search crews to north central Texas early yesterday.

Bush was told that NASA lost touch with Columbia but there are no signs at this point that terrorism was involved, US officials said. Columbia was launched January 16 amid extremely tight security amid heightened concerns since the September 11 attacks in the United States and due to the presence of Israeli astronaut Ilan Ramon.

Ramon, the son of an Auschwitz survivor, is a 48-year-old air force colonel who as a fighter pilot took part in several missions against Arab targets in 1973 and 1982. He was Columbia's payload specialist.

The Space Shuttle Columbia was commissioned in 1988 and was on its 28th mission.

The Columbia crew, divided into two teams, worked around the clock for 16 days to complete more than 80 experiments on the effects of weightlessness on human physiology, on the growth of crystals and proteins, and on combustion.

The experiments were drawn from proposals made by research centres, universities and private businesses, as well as suggestions sent by students from schools in Australia, China, Israel, Japan, Liechtenstein and the United States.

It was almost 17 years to the day that the Challenger shuttle exploded on January 28, 1986, killing all seven astronauts on board.

NASA would say only that it had declared a "contingency" and that

search and rescue units had been scrambled to search for any wreckage.

Citizens in the Dallas-Fort Worth area were warned not to approach any wreckage they might find.

NASA mission control in Houston said, "Any debris that is located in the Dallas-Fort Worth vicinity should be avoided and may be hazardous due to the toxic nature of propellants used on board the shuttle and should be reported to local law enforcement authorities."

In Palestine, Texas, a city southwest of Dallas that could be on a flight path that would turn up debris, police spokesman Steve Petrovich said, "We've gotten numerous calls throughout the city of some type of sonic-type boom and people seeing vapour trails."

There were numerous eyewitness reports of the shuttle apparently breaking apart, with several trails seen asit headed toward Earth.

Dan Archer told local television from Waco, Texas. "I kept tracking the shuttle and then it appeared to be breaking up."

Another witness told CNN: "We heard a rumble. It eventually got to where our house was shaking ... it lasted for about a minute and a half. It felt like you lived with a railroad in your backyard."

Weather forecasters had predicted clear skies and light winds at the Kennedy Space Centre. NASA headquarters said there had been no reported difficulties from the shuttle before it lost contact with ground control.