SEOUL, Dec 18 (AFP) - North Korea fired an "unidentified ballistic missile" Sunday, Seoul's military said, days after Pyongyang announced a successful test of a solid-fuel motor for a new weapon system.

"North Korea fires an unidentified ballistic missile into the East Sea," South Korea's Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan.

Sunday's launch came days after North Korea tested a "high-thrust solid-fuel motor", with state media describing it as an important test "for the development of another new-type strategic weapon system".

Despite heavy international sanctions over its weapons programmes, Pyongyang has built up an arsenal of intercontinental ballistic missiles (ICBMs).

All its known ICBMs are liquid-fuelled, however, and North Korean leader Kim Jong Un has placed strategic priority on developing solid-fuel engines for more advanced missiles.

Kim said this year that he wants North Korea to have the world's most powerful nuclear force, and declared his country an "irreversible" nuclear state.

The wishlist he revealed last year included solid-fuel ICBMs that could be launched from land or submarines.

The latest motor test was a step towards that goal, but it is not clear how far North Korea has come in the development of such a missile, analysts said.

Pyongyang has conducted an unprecedented wave of weapons tests this year, including the firing of its most advanced ICBM.

The United States and South Korea have warned for months this could come in North Korea conducting its seventh nuclear test.