Seoul, June 2 : North Korea Thursday fired four short-range missiles over the Sea of Japan, according to South Korean reports.
The fourth missile was launched at 9.20 p.m. (1220 GMT), the Yonhap News Agency reported, citing the defence ministry in Seoul.
Like the previous three launches, which began at 5.20 p.m., the fourth took place from North Korea's southeast coast near the city of Wonsan, it said.
The missiles were believed to be ground-to-ship missiles.
Earlier, the ministry had confirmed the firing of the first two missiles over the sea, also known as the East Sea.
Both missiles with ranges of 120 to 160 km fell into the ocean about 100 km from the coast, the South Korean public broadcaster KBS reported.
The missile tests were thought to be part of routine manoeuvres by the North Korean military but, because of tensions on the Korean Peninsula, a new demonstration of North Korea's military strength was not being ruled out, KBS quoted a defence ministry official in Seoul as saying.
The launches came as a nuclear test, earlier missile firings and threats from Pyongyang as well as tightened UN sanctions against North Korea have ratcheted up tensions with the country.
Meanwhile, talks between North and South Korea on the future of their final remaining large joint economic project began and ended Thursday without any tangible results.
North Korea continued to demand large increases in the rent paid for the joint industrial park in the North Korean border city of Kaesong as well as in the wages paid to North Korean workers employed by South Korean firms there, the Unification Ministry in Seoul said.
South Korea had been demanding that its neighbour release a South Korean worker detained there for allegedly criticizing the totalitarian regime in Pyongyang.
Neither side at the talks was able to narrow their differences, the ministry said. The negotiations ended hours before the missile launches.
North Korea had last fired a series of short-range missiles in the week after its May 25 nuclear test, and earlier Thursday, a South Korean newspaper reported that North Korea appeared to be preparing to carry out within days a new barrage of missile tests, including short-range ballistic and ground-to-ship rockets.
The ballistic missiles were believed to be Scud-B missiles with a range of 340 km or Rodong rockets, whose 1,300-km range was likely to be shortened to 400 km for the upcoming tests, the JoongAng Ilbo said, citing an unnamed intelligence source.
It appeared that North Korea would conduct the tests from two launch sites on its east coast, the source said after North Korea had warned ships to stay clear of the area.
Besides the short-range missiles it launched in May, North Korea in April launched what it said was a satellite, but its neighbours and the United States said it was a test for a long-range missile that potentially could hit the western US.
The international community condemned the test and launches, and in response, the UN Security Council approved a resolution last month that bars North Korea from developing and testing missiles and other weapons of mass destruction.
In return, North Korea said it would step up its defences.
South Korea said North Korea last month barred ships from off parts of its east coast because of a military exercise. The ban is in effect until July 10. Japan said it was also informed of it.
Observers said they saw the ban as potential preparations for new missile tests.