By Andrea Shalal

WASHINGTON, March 16 (Reuters) - Two U.S. senators are asking Defense Secretary Ash Carter to jumpstart a Pentagon initiative to develop a new U.S. rocket engine that could replace the Russian-built RD-180 motor, which powers a key rocket used to lift satellites into space.

Senators James Inhofe, an Oklahoma Republican, and Bill Nelson, a Florida Democrat, both members of the Senate Armed Services Committee, said they were concerned that the Defense Department and the Air Force were not complying with a law that seeks development of a U.S. alternate engine by 2019.

The senators said the Air Force was not using $220 million appropriated by Congress in the fiscal 2015 budget for work on a new U.S. engine, and had not budgeted enough funds to meet the 2019 deadline.

Congress is pushing for development of a U.S. engine given rising tensions between the United States and Russia over Moscow's annexation last year of the Crimea region of Ukraine.

In a letter dated March 10, Inhofe and Nelson said Congress made clear with passage of the 2015 defense policy bill that it wanted to see a competition to develop a new engine that would be available for all launch providers.

Officials from United Launch Alliance, the Air Force, and Space Exploration Technologies, a privately held company that is seeking certification to carry out some of the launches now done exclusively by ULA, will testify about the issue before the House Armed Services Committee on Tuesday.

The senators said the law mandated a focus on the engine, not the broader launch systems, amid signs that the Air Force wants to buy launch services rather than put money toward a new engine.

The Air Force is finalizing its plans for ending reliance on the Russian-built RD-180 engine, which powers the Atlas 5 rocket, one of two rockets used by United Launch Alliance, a joint venture of Lockheed Martin Corp and Boeing Co , to launch U.S. military and intelligence satellites.

Last month, the Air Force issued a request for information that indicated it was focused more on buying commercial launch services than development of a new engine.

Launch providers such as ULA, SpaceX and Orbital ATK welcomed the Air Force document, but it disappointed companies such as GenCorp's Aerojet Rocketdyne, which has been working with Dynetics Inc, a private research group, to research a new engine.

(Reporting by Andrea Shalal; Editing by Ken Wills)