The latest test in contravention of UN sanctions is part of Pyongyang’s ongoing effort to modernise its weaponry.

The country “has developed new-type high-thrust solid-fuel engines for intermediate ballistic missiles again, which are of important strategic significance,” the official Korean Central News Agency (KCNA) reported on Wednesday.

“The test provided a sure guarantee for reliably accelerating the development of the new-type IRBM system,” KCNA said, adding that the tests took place on November 11 and 14.

Military analysts say solid-fuel missiles are easier and safer to operate, and require less logistical support, making them harder to detect than liquid-fuel weapons.

North Korea has carried out a slew of weapons tests in recent years, including its first solid-fuel intercontinental ballistic missile (ICBM) and a “new type” of submarine-launched ballistic missile (SLBM), as leader Kim Jong Un steps up his efforts to modernise the country’s military. The country is banned from carrying out ballistic missile tests under UN sanctions.

North Korea’s General Missile Bureau said the recent tests were essential for enhancing the strategic offensive capabilities of the military in light of “the grave and unstable security environment facing the country” and the “vicious” collusion of its enemies, KCNA said.

The announcement came as a Russian delegation led by Moscow’s natural resources minister Alexander Kozlov was in Pyongyang to hold talks on issues from trade to economy, science and technology.

The two countries’ growing military cooperation has been a source of concern, with United Nations’s member states enforcing the Korean War armistice saying this week they were concerned that Russia and China were helping North Korea expand its military capabilities by enabling Pyongyang to evade UN sanctions.

The United States has also said North Korea is sending weapons to Russia for use in Ukraine and that Moscow is providing Pyongyang with technical military support.

North Korea is also trying to put a military spy satellite into orbit – an effort at which it has already failed twice – and South Korea has said Moscow is providing it with the space technology to help.

North Korea and Russia have denied any arms deals, although they have promised to deepen military cooperation.

Kim Jong Un travelled to eastern Russia in September where he held a summit with Russian President Vladimir Putin at the Vostochny Cosmodrome and later toured weapons factories as well as the naval base in Vladivostok.