South Korea's defence ministry said Monday it had detected signs that North Korea was preparing a ballistic missile launch, as Japan reportedly put its military on intercept alert.

UN resolutions ban North Korea from any use of ballistic missile technology, although it regularly fires short-range missiles into the sea off its east coast.

Tensions have been running high on the divided Korean peninsula since the North's fourth nuclear test in January, followed by a long-range rocket launch the following month.

And in recent weeks Pyongyang has voiced anger at Seoul's refusal to accept repeated offers of military talks to de-escalate the situation.

"We are tracking signs that North Korea is preparing a ballistic missile test and are maintaining combat readiness," a defence ministry official told AFP.

The official did not specify the missile type, but the fact that signs of a launch had been detected would point to a medium-range missile or larger.

In April the North tried and failed three times to test-fire a powerful new mid-range missile known as a Musudan.

In Tokyo, public broadcaster NHK said the Japanese government had put its military on alert for a possible launch, with orders to intercept any missile that threatened Japanese territory.

Under the order, the Self-Defence Forces will deploy Aegis destroyers equipped with missile interceptors offshore and PAC-3 surface-to-air anti-ballistic missiles, NHK said.

A Japanese defence ministry spokeswoman declined to confirm the news reports.

The Musudan is believed to have a range of anywhere between 2,500 and 4,000 kilometres (1,550 to 2,500 miles). The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.

The missile has never been successfully flight-tested.

The three failures in April were seen as an embarrassment for the leadership, coming ahead of a party congress which was meant to celebrate the country's achievements.

During the congress, North Korean leader Kim Jong-Un personally extended the offer of military dialogue with the South.

The proposal was repeated several times by the North's military, but Seoul dismissed all the overtures as insincere "posturing" given Kim's vow at the same congress to push ahead with the country's nuclear weapons programme.