North Korea Sunday test-fired a short-range missile off its east coast, its fourth in two days, despite pleas from South Korea and the UN chief to halt the launches at a time of high tensions.

The guided missile was fired into the East Sea (Sea of Japan) on Sunday afternoon, a defence ministry spokesman told AFP without elaborating.

On Saturday the North fired three short-range missiles off its east coast, apparently as part of a military drill.

The North's short-range missile launches are not unusual but come at a time of heightened alert on the peninsula, following Pyongyang's February nuclear test which sparked tougher UN sanctions.

Angered by the sanctions and by a joint US-South Korean military exercise, the North for weeks threatened nuclear or conventional attacks on Seoul and Washington.

The South and its US ally had earlier been watching for any test by the North of medium-range Musudan missiles. But a US defence official said early in May the two mid-range missiles had been moved from their launch site.

However South Korea's unification ministry, which handles cross-border relations, said the short-range launches also pose threats to the region and should be stopped immediately.

"We find it deplorable that the North does not stop provocative actions such as the launch of guided missiles yesterday," said unification ministry spokesman Kim Hyung-Seok, before the latest exercise.

"We call on the North to take responsible actions for our sake and for the sake of the international community."

UN chief Ban Ki-moon, speaking in Moscow, also called for Pyongyang to "refrain from" further missile tests. He said it was time for it to resume talks with the international community and reduce tensions.

The US State Department urged Pyongyang to exercise restraint, without specifically commenting on the launches.

It was unclear what type of missiles were fired Saturday and Sunday.

Seoul military officials quoted by Yonhap news agency said they may be KN-02 surface-to-surface weapons with a range of up to 160 kilometres (99 miles), or rockets of at least 300mm in calibre fired from a multiple launcher.

Park Yong-Ok, a former South Korean deputy defence minister, described the short-range missile launches as an act of "desperation" by the North's leader Kim Jong-Un, after his country's recent threats met a strong response from Seoul and Washington.

US President Barack Obama and the South's President Park Geun-Hye, at a summit this month, vowed to offer no concessions in dealing with Pyongyang.

"Such a stern response must have baffled the North greatly and Kim Jong-Un... must have turned to missile tests to seek ways out of this deadlock," Park said in a TV interview on Sunday.

Seoul said efforts to present a united front were jeopardised by last week's surprise visit to North Korea by an adviser to Japanese Prime Minister Shinzo Abe as "unhelpful" in trying to preserve a united front against Pyongyang.

Abe said Sunday he would seek talks with Pyongyang to try to settle the nagging issue of its kidnapping of Japanese, without risking his country's alliance with Washington and Seoul.

Apart from security matters, inter-Korean relations have been soured by the suspension of operations at a jointly-run industrial estate.

The Kaesong Industrial Complex, established just north of the border in 2004 as a rare symbol of cooperation, fell victim to the two months of elevated military tensions.

The North barred South Korean access to the zone and pulled out its own 53,000 workers early last month. Seoul withdrew the last of its nationals early this month.

When the South Koreans left, they loaded up cars with bundles of products, but were still forced to leave much stock behind.

The North last week rejected the South's call for talks on removing goods from the complex, calling it "a crafty ploy" to deflect blame for the suspension of operations.

"It is very regrettable that the North denigrates our offer for talks... and shifts blame for the suspension of the Kaesong complex to us," unification ministry spokesman Kim said Sunday, urging Pyongyang to come forward for talks as soon as possible.