The US isn't going to have the rocket engines it needs for its military satellites for another six years, and Russia remains a reliable partner in providing an affordable means into space, the Pentagon says.

The US Department of Defense may need to buy up to 18 Russian RD-180 rocket engines over the next six years to launch their military satellites into space, Deputy Defense Secretary Robert Work told Reuters news agency on Friday.

Saying that US needs the engines, which provide an "affordable and reliable means into space," Work added that the Pentagon "just [doesn't] see any way you can get a new engine in anything less than six years."

"And so, therefore, in the transition period, we believe strongly that we need RD-180 space engines. No more than 18 but, you know, that's our position."

In 2014, US lawmakers passed a law demanding that Washington phase out its reliance on the Russian-made rockets after 2019, following the rupture in the relations over the ongoing crisis in Ukraine.

However, in December 2015, the ban was lifted, to the outrage of Republican Arizona Senator John McCain, the main proponent of the ban.

Last month, Air Force Secretary Deborah Lee James told the Senate Armed Services Committee (which McCain chairs) that the early suspension of US use of the Russian RD-180 could cost US taxpayers up to $5 billion.

Moreover, lawmakers said that they feared that a premature phase-out could cause United Launch Alliance, a joint venture between Lockheed Martin and Boeing, to go out of business, leaving only the privately held SpaceX to lift US satellites into space.