A Nasa rover on Mars has fallen silent as a gigantic dust storm envelops the planet and blots out the sun.

Flight controllers tried to contact Opportunity late on Tuesday, but the vehicle did not respond.

The storm has been growing since the end of May and now covers a quarter of the planet.

Controllers expect it will be several more days before there is enough sunlight to recharge Opportunity's battery through its solar panels.

Nasa said the battery is likely so low that only a clock is still working, to wake the spacecraft for periodic power-level checks.

The space agency launched the twin rovers Opportunity and Spirit in 2003 to study Martian rocks and soil.

Spirit has not worked for several years. Opportunity, however, has kept exploring well past its expected mission lifetime.