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.

The retreat has angered Russia’s fiercest critics in Congress, including Senator John McCain, Republican of Arizona and chairman of the Armed Services Committee. He said that NPO Energomash, the Russian company that makes the rockets and has close ties to President Vladimir V. Putin, stood to make $300 million from sales that would otherwise come to an end.

The ban applies only to national security missions, but the Pentagon’s request has also frustrated those who have pressed to end American reliance on Russia for NASA and commercial spaceflights, at a time when relations between the two countries have become tenser than in any other period since the Cold War.

“I don’t know what the Pentagon’s position can be, except for them and the Obama administration trying to placate Putin,” Representative Duncan Hunter, Republican of California and a member of the House Armed Services Committee, said. He predicted that the legislative fight would intensify in the months ahead.

“Can you imagine the space race using Russian rockets?” he said.

The Pentagon’s position, however, has powerful support from the nation’s intelligence chief and two of the most influential defense contractors, Boeing and Lockheed Martin. And it appears to be prevailing, even as the United States has imposed a raft of financial and travel sanctions against Russian officials, including those overseeing that country’s defense industry, in order to deter further aggression by Mr. Putin in Ukraine.

In a letter to senior lawmakers dated May 11, the secretary of defense, Ashton B. Carter, and the director of national intelligence, James R. Clapper Jr., said the Pentagon would face “significant challenges” to ensuring access to space for the most delicate military and intelligence missions if the ban on the Russian engines remained in place.

When the House passed its annual defense authorization bill, it approved language that would ease the ban. The Senate Armed Services Committeerecommended a compromise that would allow the company that uses the Russian rockets, a joint venture of Boeing and Lockheed Martin called theUnited Launch Alliance, to continue to bid for additional launches through at least the rest of this decade.

The debate is an example of the seismic shift in relations between the United States and Russia, ending a quarter century of tentative and sometimes strained efforts to cooperate.

The reliance on Russian rockets for national security missions — the ones that place top-secret surveillance and communications satellites in orbit — is a remnant of the “peace dividend” that followed the Cold War. That era has now given way to one of political and military provocation.

Energomash, the Russian company that makes the rocket engines, emerged from the old Soviet space program and formed a partnership with Lockheed Martin in the early 1990s to develop new technologies. It was a time when the United States sought to foster cooperation in space, in part to discourage the proliferation of rocket technology to countries that American officials viewed warily.

Along with Lockheed Martin and Pratt & Whitney, Energomash adapted the RD-180 rocket engine for use in the Atlas series of rockets now made by the Lockheed Martin-Boeing alliance. The engine has been used in 54 launches of the latest version, the Atlas V, for commercial, NASA and military missions. In a statement, the United Launch Alliance called the RD-180 “a technologically advanced and reliable engine.”

The arrangement attracted opposition because of Energomash’s majority state ownership and, according to an article by Reuters last year, a minority share tied to one of Mr. Putin’s closest friends, the billionaireYuri V. Kovalchuk. The invasion of Crimea and Russia’s support for the armed uprising in eastern Ukraine prompted overwhelming support in Congress last year for ending the use of the RD-180 for military and intelligence missions.

“Certainly we cannot have Vladimir Putin and his cronies profit from the sale of rocket engines,” Mr. McCain said at a news conference last month.

The Pentagon and the United Launch Alliance agree on the need for rockets that are made in the United States. In testimony before the Senate in April, the secretary of the Air Force, Deborah Lee James, said the invasion of Crimea “made it abundantly clear to all of us that we have to stop relying on Russian engines.”

But she also said that halting any new purchases made after the invasion of Crimea, as the law now requires, could leave the Air Force without a viable engine for the Atlas after 2018.

Ending the reliance on the RD-180 “is not as simple as it appears,” said David A. Deptula, a retired Air Force general who runs the Mitchell Institute for Aerospace Studies in Arlington, Va. He said “underinvestment, lack of a clear vision and stifling bureaucracy” had slowed innovation of alternatives for years.

“We must always remember that this nation went to the moon in less than a decade,” he wrote in an email.

The debate over the Russian engines has become entangled in an emerging rivalry among the companies vying for the lucrative business of space launches, amounting to $70 billion in contracts for military and intelligence missions alone between now and 2030, according to anestimate cited by the Government Accountability Office.

The United Launch Alliance, formed in 2006, has a monopoly on military and intelligence contracts, but it faces competition from Space Exploration Technologies Corporation, or SpaceX, founded by Elon Musk, the entrepreneur behind PayPal and Tesla. SpaceX has developed its own rocket, called the Falcon 9, which has carried out 18 successful missions for NASA and private companies, including the first private resupply of the International Space Station. After months of review, negotiations and a lawsuit that was settled last year, the Air Force last month certified the Falcon 9 for use in national security missions, ending United Launch Alliance’s monopoly.

According to Ms. James, SpaceX can now compete for two launches scheduled this year and seven more planned for the next two years. The Air Force announced this week that it had opened bidding on the first of those, with applications due by June 23.

SpaceX has argued that its rockets can carry out the launches at a lower cost than the two used by the United Launch Alliance, the Atlas V and the Delta IV. In a written statement, Mr. Musk called the certification “an important step toward bringing competition” to national security missions.

The United Launch Alliance has pledged to build a new rocket, called the Vulcan, without using the Russian engine, but its first test is not scheduled until 2019 and its certification is not expected until 2022. In their letter, Mr. Carter and Mr. Clapper said the ban on new acquisitions of the Russian engines would mean that the alliance could soon exhaust its supply of Atlas V rockets, leaving only the more expensive Delta IV, which is being phased out.

The debate now is over how many more missions the United Launch Alliance should be allowed to conduct with the Russian engines. The company has ordered 29 engines, 15 of which are paid for and planned for use. The Pentagon wants the company to be able to use the rest. The House legislation would do that, while the Senate version would allow the alliance to buy more engines only if it wins bids, presumably against SpaceX.

If the ban remains, SpaceX could end up as the sole company able to bid for some launches in the coming years. That would recreate the monopoly that the United Launch Alliance enjoyed and that the Pentagon, SpaceX and others have sought to end. Mr. Carter and Mr. Clapper said they wanted to maintain “an environment where price-based competition is possible.”

Critics of the Pentagon’s efforts said that with SpaceX’s certification, the argument for continuing to use the Russian engine was little more than a concession to Boeing and Lockheed Martin — and one that ultimately would provide benefits to Russia.

“Some of our biggest defense companies are lobbying on behalf of the Russians,” Mr. Hunter said. “That’s a strange position for the defense industry to have.”