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North Korea said Saturday it has successfully tested the booster capability of a new intercontinental ballistic rocket engine that could give it the power to target the United States with nuclear strikes.

"We have learned from North Korea's official Central News Agency's news that North Korea has conducted another test in securing another technical step in developing its intercontinental ballistic rocket (ICBM) engine," said a South Korean military official from the Ministry of National Defense.

"We are closely monitoring North Korea's further efforts in ICBM development as well as any movement towards missile launch," he said.

The test, if true, would mean that North Korea made another step towards an ICBM program that could be used in an attack on U.S. soil, according to a North Korean expert in South Korea.

"North Korea seems to be trying to tell the world that their repeated recent threats about launching nuclear strikes on the U.S. are not bluffing," Uk Yang, a North Korea expert at Korea Defense and Security Forum. "By disclosing their ICBM technology bit by bit and today's test, if successful, confirms that they have booster technology for ICBM missile."

North Korea conducted a nuclear test Jan. 6, followed in February by a satellite rocket launch that was widely seen as a disguised ballistic missile test.

In March, North Korea made nuclear threats during a propaganda push in opposition to joint U.S.-South Korean military exercises.