MOSCOW - Six astronauts were forced to take refuge aboard the International Space Station's "lifeboat" crafts on Tuesday, bracing for the threat of a collision with floating space debris, the Russian space agency said.

"A situation arose linked to unidentified 'space trash' passing very close to the space station. The crew was told to take their places aboard the Soyuz spacecraft," Roskomos said in a statement.

The space junk narrowly missed the vulnerable orbiting station by just 250 metres (820 feet) on Tuesday as astronauts were prepared to jump ship, the RIA Novosti news agency cited an official as saying.

It is not the first time space station crews have scrambled for shelter from accumulated space junk. Crews are routinely put on alert to prepare to move out of harm's way.

Three crew members were forced briefly to evacuate the space station in an incident in March 2009.

The station -- a $100 billion project of 16 nations under construction about 220 miles (350 km) above the earth since 1998 -- is currently manned by three Russians, two Americans and a Japanese astronaut.

Only 10 percent of all objects in Earth's orbit are satellites, while the rest is trash: spent rocket stages, defunct satellites, acceleration blocks and other debris, a spokesman for the agency told state news agency Itar-tass.

Even small objects present a danger to astronauts in orbit, where trash the size of an egg can travel at dangerous speeds.

The minefield of space debris is a growing hazard with ever more satellites in orbit, and one of the most important challenges of future orbital ventures, industry expert Vladimir Gubarev told Reuters.

"Everything is spaced out just some 100 metres from each other. One satellite gets in the way of the next. It's way too crowded," said Gubarev, a renown space journalist and the Soviet spokesman for the joint Apollo-Soyuz mission in 1975.