north korea
North Korean leader Kim Jong Un supervised a ballistic rocket launching drill of Hwasong artillery units of the Strategic Force of the KPA on the spot in this undated photo released by North Korea's Korean Central News Agency in Pyongyang, March 7, 2017. REUTERS

Amid growing concerns about North Korea developing a missile that could hit the U.S. mainland, an American missile expert said the engine for Pyongyang's newly developed intermediate range ballistic missile is suspected to have been sourced from foreign countries like Ukraine or Russia.

Michael Elleman, senior expert for missile defense at the International Institute for Strategic Studies (IISS), told 38 North Thursday that the missile's engine was different from the one used in the Musudan intermediate-range ballistic missile that Kim Jong Un's regime was believed to be working on as a step toward an intercontinental ballistic missile (ICBM) development.

Read: Kim Jong Un Supervises Test-Fire Of New Anti-Aircraft Weapon

On May 14, North Korea successfully test-fired the Hwasong 12 missile, demonstrating a big move in its pursuit of an ICBM capable of reaching the continental U.S.

According to Pyongyang, the latest missile was fired at the highest angle and the missile reached the maximum altitude of more than 1,310 miles kilometers before landing in the East Sea, 500 miles away. Experts said the missile could have flown about 2,800 miles if it had been fired at a normal angle, according to Yonhap News.

"This, to me, is a big deal," Elleman said during a briefing organized by 38 North. "It's highly unlikely that it was indigenously designed, developed and produced. My working hypothesis at this point, given the performance of the engine, its appearance, it likely is derived from an old Russian engine called the RD-250."

The expert also said that the missile engine could have been sourced from Ukraine.

"What is important here is it comes from a different manufacturer than all the other engines that we've seen, and missiles we've seen, used in North Korea," he said. "We could be in for surprises in the future. It's very difficult to bound their capabilities and what might occur in the future, because we don't know if they have access to new and improved engines."

Elleman said that the Hwasong 12 is the "first step" toward developing an ICBM capable of striking the U.S. While many experts have earlier claimed that North Korea is far from developing a missile that could hit the U.S., Elleman said that the reclusive country would need at least three years to conduct enough tests if they want to have 75 percent confidence in an ICBM.

"So I don't foresee an operationally viable ICBM before 2020, maybe 2021," he said. "For emergency use, they could shorten that timeframe, but at the risk that one or a large fraction of their missile force would fail."

North Korea has been conducting missile launches, increasing tensions in the Korean Peninsula despite receiving tough sanctions and warnings from the United Nations.

North Korea warned the U.S. about its recent military drills with South Korea as well as its missile defense system, the Terminal High Altitude Area Defense system (THAAD), installed in South Korea in April. North Korea has accused the U.S. of escalating tensions with its actions and refers to its missile launch tests as measures to safeguard its country.

“By relentlessly bringing in a number of strategic nuclear assets to the Korean peninsula, the U.S. is gravely threatening the peace and safety and driving the situation to the brink of nuclear war,” North Korean officials said in a statement in April. “This has created a dangerous situation in which thermo-nuclear war may break out at any moment.”