SEOUL (Reuters) - North Korea fired two short-range ballistic missiles into the sea on Thursday in defiance of U.N. Security Council resolutions, as South Korean and U.S. forces conducted massive war games.

North Korean leader Kim Jong Un meets scientists and technicians in the field of researches into nuclear weapons in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang March 9, 2016. REUTERS/KCNA

The North also announced it had scrapped all agreements with the South on commercial exchange projects and would “liquidate” South Korean assets left behind in its territory.

North Korea has a large stockpile of short-range missiles and is developing long-range and intercontinental missiles. Thursday’s missiles flew about 500 km (300 miles) into the sea, off the east coast city of Wonsan and probably were part of the Soviet-developed Scud series, South Korea’s defence ministry said.

United Nations Secretary-General Ban Ki-moon responded to the launch by calling on Pyongyang to “cease destabilising acts,” his spokesman said, adding that Ban remained “gravely concerned” by the situation on the Korean peninsula.

Japan, within range of the longer-range variant of Scud missiles or the upgraded Rodong missiles, lodged a protest through the North Korean embassy in Beijing, Japan’s Kyodo news agency reported.

A Pentagon spokesman, Commander Bill Urban, said the U.S. Defense Department was aware of the reports of the missile launches. “We are monitoring the situation closely,” he said.

North Korea often fires short-range missiles when tensions rise on the Korean peninsula. Pyongyang gets particularly upset about the annual U.S.-South Korea drills, which it says are preparations for an invasion.

The United States and South Korea remain technically at war with North Korea because the 1950-53 Korean War ended in an armed truce instead of a peace agreement.

Around 17,000 U.S. military personnel are participating alongside some 300,000 South Korean troops in what South Korea’s Defence Ministry has called the “largest-ever” joint military exercises.

North Korea warned on Sunday it would make a “pre-emptive and offensive nuclear strike” in response to the exercises.

‘LIQUIDATING’ ASSETS

After Thursday’s missile launches, North Korea announced it would “liquidate” South Korean assets left behind in the Kaesong industrial zone and in the Mount Kumgang tourist zone.

South Korea protested the move as “totally unacceptable” but did not say what it could do to recover the assets that it valued in excess of 1.4 trillion won ($1.17 billion).

Seoul suspended operations in the jointly run zone last month as punishment for the North’s rocket launch and nuclear test.

Mount Kumgang was the first major inter-Korean cooperation project. Thousands of South Koreans visited the resort between 1998 and 2008. Seoul ended the tours in 2008 after a North Korean soldier shot dead a South Korean tourist who wandered into a restricted area.

North Korea is also livid about stepped-up United Nations sanctions adopted last week following its recent nuclear test and long-range missile launch.

South Korea’s foreign ministry said Thursday’s missile launches violated a series of U.N. Security Council resolutions and that it would refer the matter to the Council’s sanctions committee mandated to enforce the resolutions.

Chinese Ministry of Foreign Affairs spokesman Hong Lei described the situation on the Korean peninsula as “complex and sensitive.”

“All sides should stop their provocative words and deeds to avoid a further rise in tensions,” he said.

MINIATURISED WARHEADS

North Korean leader, Kim Jong Un was quoted in state media on Wednesday as saying that his country had miniaturised nuclear warheads to mount on ballistic missiles. He called on his military to be prepared to mount pre-emptive attacks against the United States and South Korea.

It was Kim’s first direct comment on the technology needed to deploy nuclear missiles, although State media have previously made the claim, which has been widely questioned and never independently verified.. North Korean state media released photographs they said showed Kim Jong Un inspecting a spherical miniaturised warhead.

South Korea’s defence ministry said it did not believe North Korea had successfully miniaturised a nuclear warhead or deployed a functioning intercontinental ballistic missile.

The Pentagon said this week it had not seen North Korea demonstrate a capability to miniaturise a nuclear warhead a said on Wednesday the department was working on U.S. ballistic missile defences to be prepared.

North Korea conducted its fourth nuclear test on Jan. 6 but its claim to have set off a miniaturised hydrogen bomb last month has been disputed by the U.S. and South Korean governments and experts, who said the blast was too small to back it up.