A pair of NASA astronauts will step out on a previously unscheduled spacewalk as early as Monday to fix a piece of motorized equipment that stalled outside the International Space Station.

The mobile transporter rail car -- used to position the station's robotic arm -- stalled on Wednesday just four inches (10 centimeters) from where it began its move, said the US space agency in a statement.

The Canadian-made robotic arm is used to reach out and grab approaching US cargo ships and was most recently used for the arrival of Orbital ATK's Cygnus spacecraft on December 9.

Efforts to correct the problem robotically from mission control in Houston failed.

Astronauts Scott Kelly and Tim Kopra will float outside the orbiting lab to move the rail car "so it can be latched in place ahead of the Wednesday, December 23 docking of a Russian cargo resupply spacecraft," NASA said.

The Progress spacecraft docks automatically so it doesn't need the robotic arm, but a NASA spokeswoman said it is important for the mobile transporter to be latched in place prior to Progress' arrival.

"It's currently not latched since it stalled while they were trying to move it," she told AFP.

The cause of the stall remains unclear, but "experts believe it may be related to a stuck brake handle," according to ISS mission integration and operations manager Kenny Todd.

A time for the spacewalk has not been announced, but NASA said it will happen no earlier than Monday, or perhaps Tuesday.

A final decision will be made after a mission management meeting on Sunday.