North Korea's rapid rate of success with long-range missiles can be attributed to a powerful rocket engine it acquired from the black market in Ukraine and Russia, according to a study published by the International Institute for Strategic Studies.

Specifically, North Korea got its hands on a "high-performance liquid-propellant engine," according to the study authored by Michael Elleman.

"It’s likely that these engines came from Ukraine — probably illicitly," Elleman told The New York Times. "The big question is how many they have and whether the Ukrainians are helping them now."

North Korea's acquisition of the Russian-designed rocket engine, called the RD-250, also suggests the intelligence community was asleep at the wheel, given the bevy of U.N. sanctions placed on the rogue nation and the monitoring that is supposed to accompany them.

Regardless, North Korea acquired the technology it needed, and its quick success caught the U.S. by surprise.

"No other country has transitioned from a medium-range capability to an ICBM in such a short time," Elleman wrote in the study.

The Times reported that investigators have zeroed in on a deteriorating missile factory in Dnipro, Ukraine, as the source of the engines that have propelled North Korea's ICBM program.

The technology has allowed North Korea to quickly advance but it has a lot of work to do with its Hwasong-14 program, especially with its navigational and guidance systems, Elleman wrote in the study.

Elleman wrote that Pyongyang could deploy the Hwasong-14, its ICBM, in 2018, "but at the risk of fielding a missile with marginal reliability."

Plus, Hwasong-14 can only reach the Pacific Coast.

North Korea would need time to build a new missile that could reach the U.S. mainland, Elleman wrote. But with the RD-250 technology, it's only a matter of time.

"It is not too late for the U.S. . . . to negotiate an agreement that bans future missile testing, and effectively prevents North Korea from perfecting its capacity to terrorize America with nuclear weapons. But the window of opportunity will soon close," Elleman concluded.