North Korea has carried out another test of a rocket engine that U.S. officials believe could be part of its program to develop an intercontinental ballistic missile, officials told Reuters on Monday.
The latest test follows one earlier this month, and is another sign of Pyongyang's advancing weapons program.
It comes amid mounting U.S. concerns about additional missile and nuclear tests, potentially in the near future.
Several U.S. officials, speaking on the condition of anonymity, said the test took place on Friday night and the engine could possibly be used in an intercontinental ballistic missile (ICBM).
Earlier this month North Korean leader Kim Jong Un said his country had conducted a test of a new high-thrust engine at its Tongchang-ri rocket launch station, saying it was "a new birth" of its rocket industry.
At the time, North Korea's official media said the engine would help it achieve world-class satellite launch capability, indicating the test was of a new type of rocket engine for long-range missiles.
Kim also has said North Korea is close to an ICBM test-launch.
North Korea has been testing rocket engines and heat-shields for an ICBM while developing the technology to guide a missile after re-entry into the atmosphere following a liftoff, experts have said.
Once fully developed, a North Korean ICBM could threaten the continental United States, which is around 9,000 km (5,500 miles) from the North.
ICBMs have a minimum range of about 5,500 km (3,400 miles), but some are designed to travel 10,000 km (6,200 miles) or farther.
The reclusive state has conducted five nuclear tests and a series of missile launches in defiance of U.N. resolutions.
U.S. Secretary of State Rex Tillerson said during a visit to the region that a military response would be "on the table" if Pyongyang took action to threaten South Korea and U.S. forces.
The Trump administration is still deliberating its policy on North Korea, but appears to be giving priority to less risky options than pre-emptive military strikes.
It is considering sweeping sanctions aimed at cutting North Korea off from the global financial system as part of a broad review of measures to counter Pyongyang's nuclear and missile threat.