WASHINGTON (AFP) - Nasa on Wednesday hailed the first-ever landing of a spacecraft on a comet as a "breakthrough moment" in the history of space exploration.

The European Space Agency's Philae lander detached from the Rosetta spacecraft and touched down on the comet 67P/Churyumov-Gerasimenko, in a complicated manoeuvre that some experts likened to a bullet meeting another bullet in space.

"We congratulate ESA on their successful landing on a comet today," said John Grunsfeld, astronaut and associate administrator for Nasa's Science Mission Directorate.

"This achievement represents a breakthrough moment in the exploration of our solar system and a milestone for international cooperation."

He also noted that three US space agency instruments are on board Rosetta to map the comet's nucleus and search for signs of water.

"We are proud to be a part of this historic day and look forward to receiving valuable data," he said.

The 100kg lander separated from its mother ship, Rosetta, after a trek lasting a decade and covering 6.5 billion kilometres.

But the announcement was soon followed by worries that Philae may have landed in soft material and was not properly attached.

Watch the video of the Rosetta Mission here.