Seoul : North Korea fired a sixth short-range missile into the Sea of Japan on Monday, defying warnings from UN chief Ban Ki-moon and South Korea after a flurry of similar tests at the weekend.

The latest firing was confirmed by the South's Joint Chiefs of Staff (JCS), who said it was unclear if the North was testing guided missiles or rockets from multiple launchers. "North Korea launched two projectiles on Monday - one in the morning and the other in the afternoon," a JCS spokesman told AFP.

Such drills are not unusual but they come as the Korean peninsula is only just emerging from a period of particularly elevated military tensions triggered by the North's nuclear test in February.

In a statement on Monday, Pyongyang angrily rejected criticism that the missile exercises were a deliberate attempt to kick off a fresh cycle of tensions.

"Military training ... is the indisputable right of any sovereign nation," the North's Committee for the Peaceful Reunification of Korea said. "Viciously taking issue with our military's rocket firing training ... is an unacceptable challenge and a wanton provocation," it said.

North Korea fired three short-range guided missiles off its east coast on Saturday and another on Sunday.

South Korea had labelled the weekend tests "deplorable", while UN chief Ban Ki-moon urged Pyongyang to exercise restraint. "It is time for them to resume dialogue and lower the tensions," Ban said in Moscow on Sunday.

North Korea argues that the real provocation is coming from South Korea and the United States, which have carried out a series of small and large-scale joint military drills in recent months.

The joint exercises have included the use of nuclear-capable B2 stealth bombers and, most recently, the participation of the nuclear-powered aircraft carrier USS Nimitz.

On Monday South Korea's President Park Geun-Hye's top security advisor, Kim Jang-Soo, again urged Pyongyang to desist from any more drills.

"Whether it's just a test or a show of force, the North should not get involved in actions that create tension," Kim said.

At one point, North Korea had been primed to test a pair of medium-range missiles, but US intelligence said the weapons were removed from their launch pads in early May.

Meanwhile, China has called on Pyongyang to intervene over North Koreans detaining a Chinese fishing boat, it said Monday, after the vessel's owner said armed hijackers had demanded a ransom for 16 sailors.

Armed North Koreans seized the boat in waters between the two countries on May 6 and demanded 600,000 yuan ($98,000) for the men's release, its owner Yu Xuejun told AFP, adding they were probably from the Pyongyang's military.

The incident is the latest strain in the relationship between the neighbours.

Beijing is Pyongyang's sole major ally and its key provider of aid and trade, but China said it "firmly opposed" the nuclear-armed North's atomic test in February.

China has stayed in "close communication" with North Korea over the incident, foreign ministry spokesman Hong Lei said at a regular press briefing.

"We have through the relevant channels called on the DPRK to properly deal with this case, and protect the safety and legal rights of the detained fishermen," he added.

The state-run Global Times on Monday quoted Jin Qiangyi, director of the Asian Studies Centre at Yanbian University, as saying it was "possible that the nuclear state is taking revenge on China" after the UN imposed sanctions over the test.

Beijing's "low-key" approach to such incidents in the past "has been taken advantage of by North Korea to infringe upon Chinese fishermen's interests", he added.

The detention comes a year after the return of 29 fishermen also kidnapped by unidentified North Koreans who had demanded a 1.2 million yuan ransom.

Those sailors were returned without ransom after the foreign ministry said it had contacted North Korea in an effort to resolve the case, the state-run Xinhua news agency reported.

Yu had reported the latest incident to Chinese authorities, he said, but later posted details on the Internet out of frustration over an apparent lack of official action.

Xinhua on Sunday quoted an official at the Chinese embassy in Pyongyang as saying that Beijing had called on North Korea to "release the boat and the fishermen as soon as possible".