A planned launch to take new crew to the International Space Station (ISS) had been postponed by at least a month due to a rocket defect, Russia's Federal Space Agency Roscosmos said Tuesday.

An air leak was found in the Soyuz TMA-04M rocket's reentry vehicle, Roscosmos head Vladimir Popovkin said.

"Apparently, (the launch) will be postponed by one month until late April," he said.

The cause of the leak was still unknown, but an investigation will wrap up within a week, he said.

Roscosmos official Alexei Krasnov confirmed that the current crew in the ISS could stay past the scheduled return date of March 16, due to the delay.

The new launch date will be set after Roscosmos held a video conference with NASA officials set for Thursday, Krasnov said.

Since the retirement of the US shuttle fleet, Russia's Soyuz spacecraft are the only way for astronauts to reach the ISS, at least until around 2015.