North Korea fired another short-range missile into the Sea of Japan on Monday, the latest in a series of missile drills that have been condemned by South Korea and U.N. chief Ban Ki-moon.

The latest firing -- the fifth in three days -- was confirmed by a spokesman for the South Korean Joint Chiefs of Staff, who said it was unclear if the North was testing guided missiles or rockets from multiple launchers.

Chinese authorities were Sunday investigating claims that unidentified North Koreans hijacked a Chinese fishing boat, kidnapping 16 sailors and demanding a ransom, local media and an official said.

Armed North Koreans on May 6 hijacked the boat and escorted it towards North Korea while it was sailing in waters around 70 kilometers (40 miles) from North Korea's western coast, reports and the boat's owner Yu Xuejun said.

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 U.N. 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 defense ministry spokesman told Agence France Presse without elaborating.

North Korea on Saturday launched three short-range missiles into the Sea of Japan, apparently as part of a military drill, South Korea's defense ministry said.

U.S. and South Korean forces had been on heightened alert for a medium-range ballistic missile test in recent weeks amid tensions triggered by North Korea's nuclear test in February.

North Korea is now thought to have around twice as many mobile missile launchers as previously estimated by Seoul, a report said

Friday, quoting a state-run military analysis unit.

South Korea Thursday criticized an "unhelpful" visit to North Korea by a senior aide to Japan's prime minister, saying it weakened the united front needed to deal with Pyongyang.

Isao Iijima arrived in Pyongyang on Tuesday on a visit that clearly surprised both Seoul and Washington, which have been working closely with Tokyo on coordinating North Korea policy.

South Korea voiced regret Thursday at North Korea's decision to spurn an offer of formal talks on removing goods from a joint industrial complex closed by military tensions.

"It's very regrettable," said Unification Ministry spokesman Kim Hyung-Suk, again urging Pyongyang to negotiate over the South Korean firms forced to withdraw from the Kaesong zone, 10 kilometers (six miles) inside North Korea.

Japan's Prime Minister Shinzo Abe said Wednesday he may meet North Korean leader Kim Jong-Un if it could help resolve the long-pending issue of Pyongyang's kidnapping of Japanese citizens.

"If a summit meeting is deemed as an important means in considering ways to resolve the abduction issue, we must take it into consideration as a matter of course in negotiating with them," Abe told a parliamentary committee.

North Korea has yet to develop a nuclear warhead small enough to fit on a missile, a senior U.S. official said on Wednesday, contradicting a recent U.S. military intelligence report.

The North claimed its third atomic test staged in February involved a "miniaturized and lighter" warhead, prompting speculation that it had acquired the crucial technology to fit nuclear devices to a missile delivery system.

U.S. envoy on North Korea, Glyn Davies, said Friday that Pyongyang should expect no reward for its apparent decision to cancel an expected missile launch at a time of elevated military tensions.

"I don't think we should any longer be in the business of treating the absence of bad behavior as something that needs to be rewarded," Davies told reporters after a meeting in Seoul with his South Korean counterpart Lim Sung-Nam.