NASA engineers are still trying to fix an International Space Station cooling problem and have not yet decided whether spacewalks will be necessary, the US space agency said Monday.

The NASA team on the ground is "having some degree of success" at working on a faulty valve that has disrupted the equipment cooling system aboard the orbiting research outpost, said ISS Missions Operations Integration Manager Kenny Todd.

"We are pretty excited about that," added Todd, who said it remained unclear whether the problem would require a series of spacewalks to repair or replace the pump.

NASA issued no decision on whether the problem, discovered on Wednesday, would push back Orbital Sciences' maiden launch of its Cygnus unmanned cargo craft.

As of now, the Orbital mission is scheduled for Thursday, December 19, after being postponed by one day over the weekend.

If the Orbital attempt is put off until next year, astronauts at the ISS would begin their first repair mission spacewalk Thursday.

A similar equipment failure in 2010 required three spacewalks to fix.

"Sometimes you can figure out a way to limp along for two months, four months, six months," said Todd.

"We may or may not be able to do that for an extended period of time."

If spacewalks are called for this time, they would mark the first time astronauts have ventured outside the orbiting lab since a helmet leak brought an early end to a European astronaut's outing in July.

Italian Luca Parmitano was wearing a US-made spacesuit when a water leak flooded his helmet and risked drowning him.