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.
"We have intelligence to suggest that North Korea is not reliant on imports of engines," one US intelligence official told Reuters. "Instead, we judge they have the ability to produce the engines themselves."
The US officials did not disclose any details of what underpinned the assessment on the high-performance liquid-fuelled engines, called RD-250's.
Ukraine denied that it had ever supplied defence technology to North Korea.
Another US 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.
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 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.
US says open to talk if North Korea ready to disarm
Secretary of State Rex Tillerson said Tuesday, after North Korea's Kim Jong-Un postponed a threat to fire missiles towards the US territory of Guam, that Washington remains ready for talks.
But the top US diplomat said it would be up to Kim when such negotiations would begin, having previously insisted Pyongyang must demonstrate that it accepts it will have to give up its nuclear program.
"I have no response to his decisions at all at this time," Tillerson said, when asked about Kim's decision to hold off. "We continue to be interested in finding ways to get to dialogue, but that's up to him."
Speaking after the launch of a religious freedom report, Tillerson would not go into more detail as to how North Korea could demonstrate a commitment to the denuclearization of the Korean peninsula.
"North Korea would have to take some very serious steps and show us that they are serious about their interest and intent in denuclearizing the Korean peninsula," Tillerson's spokeswoman Heather Nauert said.
"It would have to do a lot more. Secretary Tillerson has talked a lot about that. He's also said 'I'm not negotiating my way back to the negotiating table,' and North Korea knows exactly what it has to do."
Related reading
Kim and Trump: A tale of two leaders
He has previously said Pyongyang must halt ballistic missile and nuclear tests for an unspecified amount of time before negotiations can begin on how to halt the stand-off and any threat of US military action.
Earlier Tuesday, the unpredictable and isolated North Korean leader had been briefed by his missile forces on a "plan for an enveloping fire at Guam," according to the North's official KCNA news agency.
But afterwards, according to KCNA, he decided to postpone the operation to "watch a little more the foolish and stupid conduct of the Yankees" and not to go ahead unless the US commits more "reckless actions."
