A US court has blocked a joint venture of Lockheed Martin and Boeing from buying Russian-made rocket engines, after private rocket operator SpaceX filed a lawsuit protesting the contract.

The preliminary injunction against the deal between the US Air Force and United Launch Systems was issued late Wednesday by Judge Susan Braden of the US Court of Federal Claims.

The ruling blocks ULS, its parent company United Launch Alliance, and the Air Force from making payments to any entity subject to the control of Russian Deputy Prime Minister Dmitry Rogozin.

That includes NPO Energomash, the Russian state-owned company that makes the rocket engines that power ULA launches of US government and national security satellites.

Rogozin heads Russia's defense industry and space program, and is on a US sanctions list over the crisis in Ukraine.

SpaceX on April 28 filed a legal protest against the contract, which guaranteed the purchase of 36 rocket cores from ULA to be used in national security launches, on the basis that it was struck without any competition from other companies.

The deal was part of the Air Force's Evolved Expendable Launch Vehicle Program, which is the fourth largest program in the defense budget at a cost of $70 billion, according to court documents.

However, it was unclear how much money was at stake in the ruling. The injunction would not apply to any purchase orders placed or money paid prior to April 30, the court documents said.

SpaceX CEO Elon Musk told reporters last week that the Air Force's awarding of a contract without allowing other companies to compete was "not right" and raised the possibility that the deal could be a violation of sanctions.

The injunction did not address the issue Musk raised of fair competition, and could be lifted if the US Treasury, Commerce Department or State Department reviews the deal and finds it does not violate sanctions.

On Tuesday, Rogozin lashed out at US sanctions on high-tech exports to Russia and said the move could endanger US astronauts at the International Space Station.

The world's astronauts rely on Russian rockets for transport to the orbiting outpost, paying tens of million per seat ever since the retirement of the US space shuttle in 2011.

"If their aim is to deliver a blow to Russia's rocket-building sector, then by default, they would be exposing their astronauts on the ISS," Rogozin said, according to the Interfax news agency.

"Sanctions are always a boomerang which come back and painfully hit those who launched them," added Rogozin on a visit to Crimea, which Russia annexed from Ukraine in March.

California-based SpaceX was the first private company to reach the ISS with its own unmanned cargo capsule in 2012, and is aiming to have a crew-carrying spaceship ready by 2017.