Japanese Prime Minister Shinzo Abe seeks an urgent meeting at the United Nations to strengthen measures against Pyongyang after North Korea fired a missile over the country.

North Korea has fired several short-range missiles into the sea off its east coast early, South Korea and the US military say, as the two allies conducted annual joint military drills that the North denounces as preparation for war.

The US military's Pacific Command said it had detected three short-range ballistic missiles, all of which failed with one of them blowing up almost immediately after launch.

The South Korean Office of the Joint Chiefs of Staff said the projectiles were launched from the North's eastern Kangwon province and flew in a northeasterly direction about 250 km into the sea.

KCNA The US military's Pacific command says the missiles were fired from North Korea's eastern coast and flew about 250 kilometres. (File photo)

Pacific Command said the missiles did not pose a threat to the US mainland or to the Pacific territory of Guam, which North Korea had threatened earlier this month to surround "in a sea of fire".

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Tensions had eased since a harsh exchange of threats between Pyongyang and Washington after US President Donald Trump had warned North Korean leader Kim Jong Un he would face "fire and fury" if he threatened the United States.

REUTERS Japanese Cabinet Minister Yoshihide Suga said North Korea's missile launch on Saturday posed no threat to Japan but they remained on high alert.

The launch is the first by the North since it test-fired a missile on July 28 that could have been designed to reach 10,000 km, putting parts of the US mainland within reach.

Japan's military said the projectiles did not appear to be objects that could threaten Japan's safety.

The South Korean and US militaries are in the midst of annual defensive drills, which the North routinely describes as preparation for invasion, that involve computer simulations of a war to test readiness and run until August 31.

SOUTH KOREAN DEFENSE MINISTRY/GETTY North Korea's latest missile test coincides with US-South Korea military exercises.

The US State Department did not immediately comment about the Saturday missile launches. Secretary of State Rex Tillerson earlier this week credited the North with showing restraint by not launching a missile since the late July ICBM test.

Tillerson had said he hoped that the lack of missiles launches or other "provocative acts" by Pyongyang could mean a path could be opening for dialogue "sometime in the near future".

Trump also expressed optimism earlier this week about a possible improvement in relations. "I respect the fact that he is starting to respect us," Trump said of Kim.

KCNA Special operation forces of the Korean People's Army (KPA), guided by North Korean leader Kim Jong Un, take part in a target-striking contest.

North Korea's state media reported on Saturday that Kim had guided a contest of amphibious landing and aerial strike by its army against targets modelled after South Korean islands near the sea border on the west coast.

In a report that lacked the North's usual belligerent threat against the United States, its official KCNA news agency quoted Kim as telling its Army that it "should think of mercilessly wiping out the enemy with arms only and occupying Seoul at one go and the southern half of Korea."

On Wednesday, Kim ordered the production of more rocket engines and missile warheads during a visit to a chemical institute of the Academy of Defence Science, an agency that he fostered to develop its ballistic missile programme.

KCNA Kim Jong Un has reportedly ordered more missiles and rockets to be made in the last week.

Diagrams and what appeared to be missile parts shown in photographs published in the North's state media suggested Pyongyang was pressing ahead with building a longer-range ballistic missile that could potentially reach any part of the US mainland including Washington.

It is also believed to be developing a solid-fuel missile that could be used for submarine launches.