In what appeared to be part of the program developing an intercontinental ballistic missile (ICBM), North Korea carried out another test of a rocket engine, according to a U.S. official. On condition of anonymity, the official said Thursday the U.S. believed the drill could be the smallest stage of an ICBM rocket engine.

Another official confirmed the test but did not give away further details on what kind of rocket component was being tested or whether it fit into the ICBM program. One official said he believed the test had taken place within the past 24 hours, Reuters reported.

Experts however say North Korea could still be years away from possessing any reliable ICBM capability, reports said.

Read: How Did North Korea React To US’ Successful Missile Interceptor Test Over Pacific Ocean?

According to a recent North Korean defector, Thae Yong Ho, who served in North Korea's embassy in the U.K., Kim Jong Un wants an ICBM test in 2017 or early 2018, a report said.

Officials in the U.S. and abroad are expressing concern over North Korea’s space launch program at Sohae on the country’s west coast. The Unha-3/Kwangmyongsong rocket launched from there in 2012 and 2016 is believed to be similar to an ICBM in many ways. An ICBM, like a space launch vehicle, can consist of two or more components, each carrying fuel and an engine. The components burn and drop off into the ocean one by one, until the payload (a satellite or a warhead) is in space. A satellite orbits the earth, whereas an ICBM warhead will have to reenter the atmosphere and land at the designated target, according to the Atlantic.

ReadNorth Korea’s ICBM Test May Mark Late Leader Kim Jong Il’s Birthday, Report Says

U.S. officials have said that Unha-3 is not likely to be used as an ICBM. But a worrying factor for the U.S. is that North Korea has showed off what appears to be two variant road-mobile ICBMs. Till now, these road-mobile missiles haven't been tested so the U.S. does not know whether they work or not. And as officials said, North Korea only has six untested ICBM launchers which means even if the country makes several missiles or warheads, an ICBM missile will not work until the launcher is tested properly, a report said.

Experts also point out North Korea's recent engine and missile tests haven't really proved to be much of a success. Michael Elleman, senior fellow for missile defense at the ‎International Institute for Strategic Studies said, as reported by the Washington Post: "For me, their biggest challenge is getting a large, two- or three-stage missile to actually work and for them to have confidence that it will work."

The U.S. Secretary of Defense James Mattis however had said last month any military solution to the North Korea crisis would be "tragic on an unbelievable scale."