WASHINGTON — After Russia annexed Crimea last year, Congress passed legislation that forced the Pentagon to stop buying Russian rocket engines that have been used since 2000 to help launch American military and intelligence satellites into space.

Now, that simple act of punishment is proving difficult to keep in place.

Only five months after the ban became law, the Pentagon is pressing Congress to ease it.

The Pentagon says that additional Russian engines will be needed for at least a few more years to ensure access to space for the country’s most delicate defense and intelligence technology.