Washington, Jun 14 (IANS): An unprecedented sandstorm on Mars is threatening the survival of NASA's solar-powered Opportunity rover, the US space agency has announced.

"We are concerned but we are hopeful that the storm will clear and the rover will be able to communicate with us," Opportunity project manager, John Callas, said late Wednesday.

The rover has been running low in power since the storm - which started on May 30 at the same point where the rover is parked - has removed its main source of energy, sunlight.

Opportunity is currently enveloped in what NASA describes as "a dark, perpetual night".

According to NASA, Opportunity appears to have automatically entered a power-saving mode in which most of its functions are suspended.

Even so, the rover has to maintain the temperature of its batteries to survive on the icy Mars.

"As long as the rover stays warm enough, and our predictions are that it will, we can go any number of days," Callas said, adding that summer on Mars is approaching and hence the temperatures will rise.

The storm has already affected a quarter of the surface of Mars, equivalent to the size of the entire American continent, and could surround the planet in a few days, as happened in 2001 and 2007.

"It is unprecedented in the pace at which it has grown and spread across the globe," Jim Watzin, the director of NASA's Mars exploration program, said at the same conference.

Scientists do not know when the storm will end and the rover will be able to generate new solar power, if its systems are not affected.

Opportunity landed on Mars in 2004 and has made discoveries about the past of the red planet.

For example, it found that at least a part of Mars had the necessary humidity conditions for mesophilic bacteria to live four billion years ago, and it also discovered that the planet used to have an acidic environment some time later.