Nasa has announced the first crewed flight by a SpaceX rocket to the International Space Station (ISS) is expected to take place in June 2019.

It will be the first manned US launch to the orbiting research laboratory since the space shuttle program was retired in 2011, forcing US astronauts to hitch costly rides aboard Russian Soyuz spacecraft.

A flight on Boeing spacecraft is set to follow in August 2019.

The timetable for both launches has already been postponed several times, but Nasa said on Thursday it would now be providing monthly updates on deadlines.

"This new process for reporting our schedule is better; nevertheless, launch dates will still have some uncertainty, and we anticipate they may change as we get closer to launch," said Phil McAlister, director of commercial spaceflight development at Nasa headquarters.

"These are new spacecraft, and the engineering teams have a lot of work to do before the systems will be ready to fly."