The United States has failed to commit to plans for an unmanned joint Mars mission with the European space agency, causing frustration abroad, top NASA officials told lawmakers on Tuesday.

At issue is a 2009 agreement to develop an ESA-US ExoMars Mission in 2016 and 2018 which would measure methane in the Martian atmosphere and collect rock and soil samples to eventually return to Earth for the first time.

The project has 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 even as lawmakers bicker over federal budget details year by year.

"It can only be done, as the Decadal Survey states, if NASA is able to reduce the cost to less than 2.5 billion dollars," said Jim Green, Planetary Science Division Director at NASA before a House subcommittee.

"We recognize in this environment of -- difficult budget situation that we are in, that compromises have to be made, decisions have to be executed that are based on the Administration's priority," he added.

"Currently OMB (Office of Management and Budget) has not officially notified NASA of cancelling Mars 16 or 18. So those discussions are ongoing. Of course, we are eagerly awaiting what the ultimate priorities will be and whether we will be able to proceed."

According to Steve Squyres, chairman of the NASA Advisory Council, the budget guidelines set forth by OMB are adequate for the mission to go forward.

"To date, however, the Administration has not committed to this partnership," he said.

"The designs of the missions are being revamped so that the Decadal recommendations can be followed and yet there is no commitment being made. I am perplexed."

Asked by a lawmaker if European space colleagues were frustrated with the US failure to commit to the project, Squyres said he felt that was true.

"It has not been my perception in talking with European colleagues that they have concluded yet that we are an unreliable partner. I sense enthusiasm," he said, however adding: "I do, sir, sense frustration."

In June, NASA asked ESA to participate in a joint review to maximize resources, and the review is currently under way.

ESA, meanwhile, has asked the Russian Space Agency if it would consider participation in the Mars 2016 and 2018 missions. Roscosmos has not yet given a formal response, according to Green.