North Korea has successfully tested new solid-fuel engines for intermediate-range ballistic missiles, state-run media said Wednesday, with the development a concern to the region as the engines can be prepared for launch more quickly than the liquid-fuel type.

Pyongyang successfully conducted the first ground jet tests of the first-stage engine on Saturday and the second-stage engine on Tuesday, the official Korean Central News Agency said. An intermediate-range ballistic missile fired from North Korea can reach Japan, host to several U.S. military bases.

The tests once again clearly showed "the reliability and stability" of the country's high-thrust solid-fuel engine designing and manufacturing technologies, KCNA said.

Undated photo shows a ground jet test of new solid-fuel engines for ballistic missiles conducted in North Korea. (KCNA/Kyodo)

The experiments are "an essential process for further enhancing the strategic offensive capabilities" of North Korean forces in the face of "the grave and unstable security environment" in which "enemies will get more vicious in their military collusion and nexus," KCNA added.

The report apparently referred to moves by the United States and its two Asian allies, South Korea and Japan. In San Francisco on Tuesday, the foreign policy chiefs of the three countries agreed to boost cooperation in tackling the North Korean missile threat.

Washington, Seoul and Tokyo are preparing to initiate a system by the end of this year that allows for the swift exchange of information on North Korean missiles.

A solid-fuel missile does not require fueling before launch like a liquid-fuel missile, making it harder for other countries to detect launch preparations and giving it a better pre-emptive strike and retaliatory capability.

Japanese Chief Cabinet Secretary Hirokazu Matsuno said at a press conference that Tokyo believes Pyongyang has developed solid-fuel ballistic missiles "to enhance its surprise attack capabilities."

The top government spokesman added Japan will continue to monitor the situation to detect any signs of further North Korean provocations, including missile launches and nuclear tests.

Meanwhile, a Russian government delegation arrived in Pyongyang on Tuesday for bilateral talks on cooperation in trade, economy, science and technology, KCNA said.

The United States, South Korea and Japan have been wary about deepening military cooperation between North Korea and Russia, with Moscow possibly providing technical advice to aid Pyongyang's missile development.

In April and July, North Korea test-launched solid-fuel Hwasong-18 intercontinental ballistic missiles.

The latest tests came as North Korea earlier this month designated next Saturday as a "missile industry day" to commemorate the launch exactly a year ago of a new-type Hwasong-17 ICBM.

Pyongyang is also expected to launch a military spy satellite soon following its unsuccessful attempts to do so in May and August. The country had said a third attempt would be made in October.

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