The RD-180 is used in the first stage of US Atlas 5 rockets, and thus far there has been no US alternative to the Russian engines.
Earlier, Congress passed a legislation barring the use of the RD-180 on future military missions. In particular, the 2015 National Defense Authorization Act directs the US Air Force to wean itself from the Russian-built propulsion system by 2019. The ban was drafted in connections with the situation in Ukraine.
A separate piece of legislation provided 220 million dollars to the Air Force earlier this year to begin development of a US alternative to the engine.
Speaking at a Senate Armed Services Subcommittee, Air Force Secretary Deborah Lee James proposed a spate of amendments to the law so that the US could buy the RD-180 engines that were ordered from Russia before the events related to Crimea's reunification with Russia. The US has yet to pay for these engines.
According to James, the amendment "will help preserve the launches of competing Atlas rockets until US alternatives to the engines are developed."
"No. It's regrettable, but guaranteed access to space is much more important," the Pentagon spokesman answered quoted by the Air Force Times.
In March 2015, the US announced a tender for space launches in connection with the ban on Russian RD-180 rocket engines. The tender will be held for 28 launches, due to be conducted between 2020 and 2024.