The announcement came as Pyongyang also disclosed a Russian delegation led by Moscow's natural resources minister Alexander Kozlov was visiting Pyongyang to hold talks on cooperation in trade, economy, science and technology.

The two countries' growing military cooperation has been a source of concern for Ukraine and its allies, especially following North Korean leader Kim Jong Un's summit with Russian President Vladimir Putin in September.

The North "has developed new-type high-thrust solid-fuel engines for intermediate ballistic missiles again, which are of important strategic significance," Pyongyang's official Korean Central News Agency reported.

The country also "successfully conducted the first ground jet tests of the first-stage engine and the second-stage engine on November 11 and 14 respectively," it added.

Experts say solid-fuel missiles typically have a higher level of operational ease and safety, compared to liquid-fuel weapons.

Solid-fuel missiles don't need to be fuelled before launch, making them harder to find and destroy, as well as quicker to use.

Testing a more technologically advanced solid-fuel missile was one of North Korean leader Kim Jong Un's major goals in the military modernisation campaign announced in his New Year report.

In April, Pyongyang said it had successfully tested its first solid-fuel intercontinental ballistic missile -- the largest, longest-range category of ballistic missile -- hailing it as a key breakthrough for the country's nuclear counterattack capabilities.

The latest engine tests "provided a sure guarantee for reliably accelerating the development of the new-type IRBM system," KCNA said Wednesday.

The advancement is crucial "in the light of the grave and unstable security environment facing the country", it added, in which the "enemies will get more vicious in their military collusion".

Historic allies Russia and North Korea are both under international sanctions -- the former for its invasion of Ukraine and the latter for its nuclear weapons and missile programmes.

South Korea has said Pyongyang is providing Moscow with arms in exchange for Russian space technology so that it can put a military spy satellite in orbit.

On Monday, the US and South Korean defence chiefs updated for the first time in a decade a key military agreement to counter Pyongyang and its growing nuclear threats.