In response to soaring tensions with Moscow, the Pentagon is reviewing whether a reliance on Russian rocket engines for US military satellite launches poses a potential national security risk.
Since 1995, the US military has used the Russian-made engines on Atlas V rockets, which are often employed to send Air Force and other satellites into space.
"In light of the current situation, the department has directed the Air Force to perform an additional review to ensure we completely understand the implications, including supply interruptions, of using the Russian-made RD-180 rocket motor in our Atlas V launch vehicle," spokeswoman Maureen Schumann said.
Although the US Defence Department regularly examines potential risks to the military's supply chain from foreign parts, Pentagon chief Chuck Hagel has ordered a fresh review of the Russian engines given the crisis over Ukraine, officials said.
After earlier reviews, officials stockpiled a supply of the Russian rocket engines and now have an inventory that would last about two years, according to Ms Schumann.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The latest US sanctions imposed against Russia over its intervention in Ukraine's Crimean peninsula do not effect any Pentagon programs, she said.
If the Atlas V with its Russian engines were scrapped, it would take up to five years and as much as a billion dollars to replace, she said.
United Launch Alliance, a joint venture of aerospace giants Lockheed Martin and Boeing, relies on the Russian engines for the Atlas rockets.
Ms Schumann called the Atlas V a "reliable, cost-effective launch vehicle with a proven record of success" and alternative launch vehicles for larger satellites would carry a higher price tag.
A senior US defence official, speaking on condition of anonymity, told AFP that it was unlikely the review would lead to any major change in the launch program or a decision to drop the Russian engines.
Separately, lawmakers in Congress have renewed demands for the Pentagon to cancel a contract for Russian-made helicopters for the Afghan government, citing Moscow's actions in Ukraine.
Complaints from lawmakers led the Pentagon last year to scale back the planned purchase of the Russian Mi-17 choppers for Afghan security forces.
