The six wheels on the robot rover had been virtually covered by sand since it got stuck on April 26.

NASA's Jet Propulsion Laboratory (JPL), based in California, said engineers had finally got the rover to move at the weekend. It posted a picture of the trace left by the wheels on its internet site.

"The Mars rover Opportunity has successfully escaped from a sand trap. JPL engineers cheered when images returned from Mars showed the rover's wheels were free," JPL said in a statement.

Opportunity and its twin robot Spirit landed on Mars in January 2004 for a mission, to take pictures of the surface of Mars and collect soil and rock samples, initially scheduled to last just three months.

But the two machines have kept going for 16 months and NASA has said they could keep operating for 18 months.

Opportunity is on the Martian Meridiani plains while Spirit is on the opposite side of the planet inside the Gusev Crater.

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