Two Nasa astronauts have celebrated Christmas Eve by venturing out of the International Space Station (ISS) for a spacewalk.

US astronauts Rick Mastracchio and Michael Hopkins floated outside the station to finish urgent cooling system repairs.

"It's like Christmas morning, opening up a little present here," Mastracchio said while checking his toolkit.

Mission Control in Houston asked Mastracchio and Hopkins to install a new pump 260 miles (418 km) above the Planet Earth.

"Expedition 38 flight engineers Rick Mastracchio and Mike Hopkins are conducting the second in a series of spacewalks to replace a degraded ammonia pump module on the International Space Station's starboard truss," Nasa said.

It said the operation, similar to that portrayed in Alfonso Cuaron's 2013 blockbuster movie Gravity, which starred George Clooney and Sandra Bullock, was scheduled to last for about six hours and 30 minutes.

The US space agency broadcast the spacewalk live on its website and tweeted live updates:

NASA's @AstroRM makes his way along the #ISS truss on the right as orbital sunrise begins over Earth on the left.

— NASA (@NASA) December 24, 2013

"It is Christmas Eve and in this holiday way of giving, we're giving you a spacewalk today," said mission controller Rob Navias in Houston.

It was the second time in four days that the astronauts left the station. They had removed the faulty pump over the weekend.

Hopkins tweeted:

Wow...can't believe that is me yesterday. Wish I could find the words to describe the experience, truly amazing

— Mike Hopkins (@AstroIllini) December 22, 2013

It was only the second Christmas Eve spacewalk in Nasa history. The first came in 1999 as part of a Hubble Space Telescope repair mission.