WASHINGTON – A congressional ban on the use of Russian engines to launch U.S. national security payloads will hamstring the U.S. Defense Department’s primary rocket maker, United Launch Alliance, in competitions for military business starting as early as this year, the U.S. Air Force has warned.

In a written response to questions from SpaceNews, the Air Force said only “a very small number” of rocket engines currently on order from Russia meet the criteria for exemption from the ban, imposed in the National Defense Authorization Act for 2015 in response to Moscow’s annexation of Ukraine’s Crimea peninsula.

In other words, the practical impact of the ban could be felt by ULA and the Air Force several years earlier than intended or previously thought.

Denver-based ULA operate two main rockets, the Atlas 5 and the Delta 4, and these vehicles will continue to launch the vast majority of U.S. national security satellites until rising competitor SpaceX enters the market with its Falcon 9 rocket, which is expected to be certified this year. The Atlas 5 is powered by the Russian-built RD-180, which is the target of the ban, while the Delta 4 is too expensive to compete effectively against the Falcon 9.

In crafting the ban, Congress carved out exemptions for engines ordered under ULA’s so-called block-buy contract, a sole-source order of a combined 36 Atlas 5 and Delta 4 rocket cores that was made in 2013. The law also exempts engines that were either paid for in full or legally committed to by ULA as of Feb. 1, 2014.

These exceptions in theory would enable ULA to continue launching national security satellites on Atlas 5 until 2019, by which time Congress expects a new U.S.-built engine to become available. ULA is pursuing two alternative concepts for a new main engine, and Congress has provided $220 million for as-yet unspecified engine development activities this year, but Air Force officials have expressed strong doubt that anything could be fielded by 2019.

For the past two weeks, both the Air Force and ULA have been openly advocating for some relief from the ban, which they have said could be more restrictive than intended.

The issue first surfaced in a Senate hearing Feb. 25 when Air Force Secretary Deborah Lee James was asked about the service’s interpretation of the law.

“We had several sections of law that may or may not have had the same intent,” James said. “They’re not necessarily together with what they instruct us to do.”

In a March 9 written response to questions, Air Force spokesman Capt. Chris Hoyler raised new questions about what, specifically, is excepted from the ban.

“While the intent of this exception language may have been to allow United Launch Alliance to use all the RD-180 engines currently on order, based on the documentation provided to the Department by ULA thus far, it appears that only a very small number of those engines actually met the statutory language,” Hoyler said. “This prohibition therefore restricts the ability of ULA to compete effectively for EELV Phase 1A and early Phase II missions inconsistent with the timelines in NDAA Section 1604 for developing additional competitive launch capabilities.”

EELV Phase 1A refers to the initial competitive phase of the Evolved Expendable Launch Vehicle program, which ULA has had all to itself since its establishment in 2006. The Air Force expects to put as many as nine missions up for bid during Phase 1A, which runs from 2015 through 2017. Under the following phase, referred to as Phase 2 in Hoyler’s statement, all missions would be put out for bid.

But Hoyler’s statement raises its own questions, in part because neither the Air Force nor ULA have offered many details of the block-buy contract or ULA’s associated RD-180 engine order from its Russian manufacturer, NPO Energomash. The engine is imported to the United States by RD-Amross, a joint venture of Energomash and United Technologies Corp.

The statement did not specify, for example, whether the engines — industry sources said the number was five — identified as exempt from the ban were on order or already in ULA’s inventory. Nor did his statement make clear how many engines are on order or how many of those are specifically tied to the block buy.

Also unclear is whether the Air Force was basing its assessment on a full or partial submission by ULA of the requested documents.

Although the Air Force cited specific concerns only about ULA’s ability to compete for future business, the statement would seem to raise questions as to whether the company has enough exempted RD-180s on hand or on order to execute the existing block-buy contract.

The Air Force did not immediately respond to queries seeking clarification of these matters.