Russia said Thursday that its delayed next manned mission to the International Space Station (ISS) would blast off May 15, 45 days later than its scheduled date, after a problem was found in testing.

"The launch is set for May 15," the head of the Russian space agency's manned programmes, Alexei Krasnov, told the Interfax news agency.

He also said that the mission currently on the ISS would stay an extra six weeks longer than scheduled and would land back on Earth on April 30. Three crew members had been set to land on March 16.

"The landing is normally held two weeks before the launch of the next mission. This time it will happen on April 30," he told Interfax.

The delay came after the re-entry capsule of the Soyuz spacecraft to be used in the mission was shown in testing not to be hermetically sealed and could not be used for safety reasons.

The re-entry capsule goes inside the spacecraft and is the portion that eventually returns the astronauts to Earth when the mission is over.

Russia now has sole reponsibility for taking US and other international astronauts to the ISS following the withdrawal of the US space shuttle but its own space programme has been hit by a string of problems in recent months.