The US space agency NASA said Monday it expects commercial operators will deliver cargo to space within months, stressing that private missions were crucial to its future human activities.

"It is months before we have commercial entities carrying cargo to the International Space Station (ISS), not years," said NASA head Charles Bolden, saying that two companies were preparing to fly final demonstration missions.

NASA this year grounded its space shuttle fleet while unveiling its new Space Launch System focused on developing a heavy-lift launch vehicle for deep space exploration.

"The Space Station will continue to be the centrepiece of our human space flight activities through at least 2020," said Bolden.

"Commercial transportation of cargo and crew remain crucial, if not critical, to our future aboard the International Space Station," he added.

The NASA chief spoke at a meeting of heads of space agencies from Russia, Europe, Japan and India at the International Astronautical Federation's annual congress.

"In November, working with our international partners, we'll start out again for the red planet with the Mars Science Laboratory, which will have the most sophisticated set of science instruments ever deployed to the planet's surface and serve as precursor as human missions to the planet," said Bolden.

Bolden and European Space Agency head Jean-Jacques Dordain said governments had not lost interest in space development despite the rise of private business in the industry.

"Using more industry and more private industry does not change the fact that at the end of the day, the government pays," said Dordain.

Russia's Federal Space Agency head Vladimir Popovkin confirmed that the next manned flight will leave for the ISS on November 14 after the freezing of launches after cargo ship's crash to Earth in August.