North Korea says it has successfully tested an engine designed for an intercontinental ballistic missile.

The new type of engine would "guarantee" the ability to launch a nuclear strike on the US mainland, the KCNA news agency said.

The test was conducted at the country's long-range missile launch site near its west coast.

It is the latest in a series of tests and launches carried out by the isolated nation.

Leader Kim Jong-un supervised the test, state media reported, during which "the engine spewed out huge flames with a deafening boom".

The country would now be able to "keep any cesspool of evils in the earth including the US mainland within our striking range," Kim was quoted as saying.

North Korea should "refrain from actions and rhetoric that further destabilise the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations," said US State Department spokesman Mark Toner.

In March, North Korea said it had developed nuclear warheads small enough to fit on ballistic missiles.

However, experts cast doubt on the claims.

March also saw North Korea threaten "indiscriminate" nuclear strikes on the US and South Korea as they held big joint military drills, which the north sees as a rehearsal for an eventual invasion.

Meanwhile, the US has imposed new sanctions on North Korea following a nuclear test in January and a satellite launch in February, which was widely seen as a test of banned missile technology.

The US has also held talks with South Korea aimed at deploying a US missile defence system to the Korean peninsula, a move strongly opposed by North Korea, Russia and China.

Beijing says the Thaad anti-missile system compromises its security and would undermine its nuclear deterrent.