The American space agency looks set to pull the plug on its joint missions to Mars with the European Space Agency.

Nasa has told Esa it is now highly unlikely it will be able to contribute to the endeavours, which envision an orbiting satellite and a big roving robot being sent to the Red Planet.

The US has yet to make a formal statement on the matter but budget woes are thought to lie behind its decision.

Europe is now banking on a Russian partnership to keep the missions alive.

A public announcement by Nasa of its withdrawal from the ExoMars programme, as it is known in Europe, will probably come once President Obama's 2013 Federal Budget Request is submitted.

This request, expected in the coming days, will give the US space agency a much clearer view of how much money it has to implement its various projects.

"The Americans have indicated that the possibility of them participating is now low - very low. It's highly unlikely," said Alvaro Gimenez, Esa's director of science.