The pair began their six-month stay in space in April

"You need to return," a Russian ground controller told them urgently - and they were safely back inside the ISS just a few minutes after leaving it.

The pressure in American Michael Fincke's main oxygen bottle had plunged, the pair were told.

His spacewalk with Russian Gennady Padalka was put off until next week.

Fincke had actually been outside less than two minutes before being ordered back in.

"The pressure in Michael's bottle is falling," said a Russian ground controller.

"Okay, Michael, we're going back in," said Padalka, the space station's commander.

The pair hurried back inside, closed the hatch and hooked up their suits to the station's air supply.

The US space agency (Nasa) said the pressure inside Fincke's suit never dropped, but the pressure inside his main oxygen tank fell sharply.

It was the second spacewalk in a row to be aborted due to a suit problem. The earlier problem involved a US suit and an earlier crew.

Construction of the complex has involved 52 spacewalks so far

Padalka and Fincke had hoped to repair one of the gyroscopes which keep the space station stable in flight.

The flight director of the Russian section of the international space station (ISS), Vladimir Solovyov, said the walk would be repeated in several days' time.

Nasa stressed that the spacemen were never in any danger during the aborted walk.

It was, however, a riskier operation than usual, as the space station was left entirely empty for only the second time since the platform has been permanently crewed. This left ground controllers having to keep an eye on the craft's systems while the men were outside the ISS.

The pair had originally planned to leave from the American hatch in Nasa spacesuits - but these developed serious cooling problems last month, forcing managers to send the two men out via the Russian hatch in Russian suits.

That more than doubled the distance to the work site, on the US side of the station.