The RD-180 has been used to power the Atlas V rocket used by the US Air Force to launch its probes as well as in NASA research programs.
Earlier, the Pentagon said that it would abandon the use of RD-180 engines only when capable analogues are developed.
According to WSJ, Deputy Defense Secretary Robert Work wrote a letter to one of the congressmen, to express the growing concerns of the US military.
Work noted that the US Air Force does not have sufficient budget capabilities to replace an abrupt ending of the use of RD-180 engines and replace them with more expensive analogues.
He warned that additional spending between $1.5 billion and $5 billion would "crowd out other important national-security investments."
Other documents submitted by the deputy defense secretary reveal that if the US ends the use of the Russian engines a number of crucial space launches for the Pentagon will be delayed for two years.
In 2014, the US Congress passed a law demanding the United States phase out its reliance on the Russian-made RD-180 rocket engines.
However, in the two years since its purchasing ban on the RD-180 engines, Congress has been forced to lift the moratorium to ensure the United States has access to space.