North Korea has fired two short-range ballistic missiles into the sea, South Korea's military has said.

It said the missiles, launched off the east coast, flew some 500km (310 miles) and fell into the water.

Shortly after the launch, Pyongyang announced it "nullifies" all inter-Korean cooperative projects and will liquidate South Korean assets in the country.

Most South Korean assets in the North are in the Kaesong industrial zone.

South Korea pulled out of the jointly-operated industrial zone in February, after North Korea's latest long-range missile launch of a satellite. At the time, the North called the shutdown "a declaration of war" and designated Kaesong as a military zone.

Miniaturisation claim

On Wednesday, North Korean leader Kim Jong-un also claimed scientists had developed nuclear warheads small enough to fit on ballistic missiles.

However, South Korea's defence ministry said it thought the North had "not yet secured miniaturised nuclear warheads".

Last week North Korea fired six rockets into the sea, using a multiple launch system. Photo: KCNA.

The claim is critical, as without miniaturisation Pyongyang cannot put its nuclear weapons on missiles - an ability many analysts think could still be several years away.

In response to the miniaturisation claims, US State Department spokesman John Kirby said of Kim Jong-un that "the young man needs to pay more attention to the North Korean people and taking care of them, than in pursuing these sorts of reckless capabilities".

The two missiles launched on Thursday morning were fired from Hwanghae Province, the South Korean military said. It added that the missiles later fell into the sea off North Korea's east coast, without providing further details.

Japan promptly lodged a protest to North Korea via its embassy in Beijing, over the latest launches, reported Kyodo news agency.

The missile launches are seen as the North's response to the UN imposing some of its strictest sanctions, after the North conducted its fourth nuclear test in January and last month launched a satellite, both in contravention of existing sanctions.

Tensions have been especially high this week as US and South Korean forces hold their annual joint military exercises known as Key Resolve and Foal Eagle.

This year they are the largest ever, with about 17,000 US personnel and around 300,000 South Korean troops participating - both significant increases on 2015, in addition to increased naval and air force assets.

The drills routinely generate tension, but this year Pyongyang threatened to launch a "pre-emptive nuclear strike of justice" against the US and South Korea.

"We will launch an all-out offensive to decisively counter the US and its followers' hysterical nuclear war moves," a newsreader on the state-run North Korean KRT news channel said at the time.