US President Barack Obama's budget proposal to be submitted next week for 2013 will cut NASA's budget by 20 percent and eliminate a major partnership with Europe on Mars exploration, scientists said Thursday.

The White House is proposing a $1.2 billion budget for the US space agency, down from $1.5 billion this year, according to Louis Friedman, a former NASA official and co-founder of the Planetary Society who was briefed on the matter.

"It's certainly dead," Friedman told AFP, referring to the ExoMars project which aimed to send an orbiter to the red planet in 2016 followed by a pair of rovers in 2018, ahead of a possible mission to return samples from Mars to Earth in the 2020s.

According to the deal NASA and the European Space Agency made in 2009, NASA would contribute $1.4 billion to the project and ESA would chip in $1.2 billion.

Friedman added that the political will in Europe and the United States was likely strong enough to push efforts toward a new deal of some kind, but the details remain to be seen.

"I don't think it is hopeless. But there is no indication that Congress is willing to help," he said.

G. Scott Hubbard, a professor of aeronautics and astronautics at Stanford University who was NASA's first Mars program director and revitalized the program after a string of failures, also said the ESA project was likely to die.

"If the budget cuts appear next Monday as the rumors suggest, then the existing NASA/ESA ExoMars collaboration may well disappear. What will replace that is unknown to me," Hubbard told AFP in an email.

NASA officials declined to confirm the details of the budget proposal but scheduled a series of press conferences to discuss it on Monday.

"Consistent with the tough choices being made across the federal government to reduce spending and live within our means, NASA is reassessing its current Mars exploration initiatives to maximize what can be achieved scientifically, technologically and in support of our future human missions," said NASA spokesman David Weaver.

The project had been named as a top priority flagship mission by the US National Academy of Sciences' Decadal Survey, which sets out a plan for NASA space exploration.