WASHINGTONWASHINGTON (Reuters) - North Korea likely has the ability to produce its own missile engines and intelligence suggests it does not need to rely on imports, U.S. intelligence officials said on Tuesday.

The assessment disputes a new study by the London-based International Institute for Strategic Studies that said that the engines for a nuclear missile North Korea is developing to hit the United States likely were made in factories in Ukraine or Russia and probably obtained via black market networks.

The New York Times cited the study on Monday. The newspaper's report said that classified assessments by U.S. intelligence agencies mirrored the IISS finding.

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"We have intelligence to suggest that North Korea is not reliant on imports of engines," one U.S. intelligence official told Reuters. "Instead, we judge they have the ability to produce the engines themselves."

The U.S. officials did not disclose any details of what underpinned the assessment on the high-performance liquid-fueled engines, called RD-250's.

Ukraine denied that it had ever supplied defense technology to North Korea. The Ukrainian factory cited in The New York Times, state-owned Yuzhmash, said it had not produced military-grade ballistic missiles since independence from the Soviet Union in 1991.

Another U.S. intelligence official said that the modifications to the RD-250 that resulted in improved reliability may have relied in part on foreign scientists recruited by North Korea or been developed by North Koreans educated in Russia or elsewhere.

Ukraine is supported by the United States in its fight against Russian-backed separatists in eastern Ukraine.

Asked about the reports that North Korea may have obtained Ukrainian-produced engines, State Department spokeswoman Heather Nauert praised Kiev's efforts to halt weapons proliferation.

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"Ukraine, though, we have to say has a very strong nonproliferation record and that includes specifically with respect to the DPRK," she said, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.

The IISS study, which coincides with tensions between Washington and Pyongyang over North Korea's nuclear weapons and missile programs, said the engines probably came from the Ukrainian Yuzhmash plant.

The study based its conclusions mainly on photographs published by North Korea of missile engines that it ground-tested in September and March and flight-tested on Hwasong-12 and Hwasong-14 missiles in May and July. Hwasong-12 is an intermediate-range rocket and Hwasong-14 is an intercontinental ballistic missile that North Korea has designed to reach the U.S. mainland.

By comparing the engines in the photographs to others, it found that they likely were modified versions of the RD-250 produced by Yuzhmash and accounted for sudden successes in North Korean missile tests following a slew of failures.

The IISS study is also being disputed by some leading independent nuclear weapons experts.

"It’s completely wrong," asserted Jeffrey Lewis, head of the East Asia Nonproliferation Program at the Middlebury Institute of Strategic Studies at Monterey, California.

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Lewis said his research team performed measurements independent from each other on the same photographs used in the IISS study and determined that they were of different sizes. They concluded that the motors for the North Korean ICBM likely were indigenously built.

Lewis also pointed to a Jan. 17, 2016, U.S. Treasury announcement of U.S. financial sanctions on Iranian firms for helping North Korea develop the engine that was tested in November and most closely resembles the Ukrainian engine.

"To me, they don’t look like those (RD-250s) engines except in the way that all (rocket) engines look," Lewis said.