Washington  - US astronauts Chris Cassidy and Tom Marshburn on Friday began an intense spacewalk to change batteries on a solar panel of the International Space Station.

During the planned seven and a half hours of work, which began at 1354 GMT, the astronauts are to replace four batteries that collect and store energy from the station's solar panels.

The spacewalkers have more work to do than originally planned after a spacewalk on Wednesday was cut short due to problems with carbon dioxide buildup in Cassidy's spacesuit. A piece of equipment that filters the gas from the air inside the suit was not working properly, but Cassidy was never in imminent danger, NASA said.

The spacesuit has been repaired and the astronauts are trained in what to do in case of an emergency, NASA said.

Wednesday's spacewalk saw Cassidy and astronaut Dave Wolf replace two batteries. They had been set to replace four of the 170- kilogramme batteries, but the work was cut short due to the spacesuit problems.

Each solar panel on the station has six batteries, which were designed to be changed after six and a half years. Because they are so large the work must be done while NASA is still operating the space shuttles that are capable of carrying large loads, the space agency said.(dpa)