The US space shuttle Columbia has disintegrated after re-entering the Earth's atmosphere, killing all seven crew. The space agency Nasa lost contact with the craft about 15 minutes before it was due to land at the Kennedy Space Center in Florida. In a televised statement, a sombre President Bush paid tribute to the six American and one Israeli astronaut. "These astronauts knew the dangers and they faced them willingly," he said.

"Because of their courage and daring and idealism, we will miss them all the more."

He said the thoughts of all Americans were with the dead astronauts' families, but he pledged that space exploration would continue.

Shuttle programme manager Ron Dittemore told a news conference Nasa could not yet say what caused the break-up of Colombia.

Thorough investigations were beginning to find out what happened during the last minutes of the flight, when some of the shuttle's sensors began to malfunction.

"Right now there is certainly a hold on future flights until we get ourselves established and understand the root cause of this disaster," he said.

Experts have suggested that the cause of the break-up was probably mechanical or structural.

Heightened security had surrounded Columbia's latest mission because of the presence of Colonel Ilan Ramon, the first Israeli in space.

There was a shocked reaction in Israel to the disaster. "The people of Israel bow their heads today in view of this great tragedy," said a statement from Prime Minister Ariel Sharon's office.

Cargo launch

Russian president Vladimir Putin said his country's co-operation in space exploration made the accident even more tragic for Russians. But the scheduled launch of a Russian cargo vessel to serve the International Space Station (ISS) will go ahead on Sunday.

Hundreds of US state troopers, police and rescue workers are searching large areas in eastern Texas for debris from the shuttle.

Columbia, which had been due to land at 0916 EST (1416 GMT) was returning from a 16-day mission orbiting the Earth and had just begun its re-entry procedure when contact was lost at about 0900 EST.

Nasa said the shuttle was about 200,000 feet up and travelling at 12,500 mph (20,000 km/h) at the time.

Television pictures showed a vapour trail from the craft as it flew over Dallas.

It then appeared to disintegrate into several separate vapour trails, and witnesses in the area said they heard "big bangs".

Texas public safety department spokesman Clive Kennelly said there were more than 2,000 debris fields, scattered from the small town of Nacogdoches about 170 miles (290 km) south-east of Dallas, to the Louisiana border

Nasa has warned that any debris found should be avoided as it could be hazardous, and that people should report such finds to the authorities.

Pieces of debris have been reported in fields and on roads, and one Nacogdoches resident, Jeff Hancock, said a metal bracket about a foot (30 centimetres) long crashed through his office roof, the Associated Press news agency reported.

PHOTO CAPTION

Shuttle Columbia: First flight: 1981; Orbiting speed: 17,500 mph; Weight: 2,000 tonnes Crew (for this mission):7 including an Israeli and an Indian-born ast