A discarded chunk of a Russian rocket came close enough to the International Space Station to force six astronauts to seek shelter in escape capsules.

Nasa said the space junk was barely close enough to be a threat but the astronauts – two Americans, three Russians and a Dutchman – woke early and went into two Soyuz vehicles ready to rocket back to Earth.

The debris came closest at 6.38am GMT. It wasn't noticed until Friday, too late to move the space station out of the way.

This is the third time in 12 years that astronauts have had to seek shelter from space junk.

The "sheltering exercise" was carried out with "due diligence and in a very precautionary and conservative fashion", a Nasa spokesman said.

"Everything went by the book and, as expected, the small piece of cosmos satellite debris passed the International Space Station without incident."

An official of the Russian space agency told the Interfax news agency that the debris passed the station at a distance of 14 miles, adding: "The cosmonauts have returned to performing their previously assigned work."