Kim Jong-un calls for 'increasing the strike ability' of North Korea's missiles

Kim Jong-un has ordered a boost to North Korea's military strike capability as tensions grow over tests appearing to show preparations for a new advanced missile system.

News of the call for "full combat posture" by Kim follows a US announcement it has seized a large North Korean cargo ship for carrying an illegal shipment of coal.

The increased tensions come amid a gridlock in dialogue after the second summit between Kim and US President Donald Trump collapsed over US demands for Pyongyang's nuclear disarmament and Kim's demands for sanctions relief.

"[Kim] stressed the need to further increase the capability of the defence units in the forefront area and on the western front to carry out combat tasks and keep full combat posture to cope with any emergency," KCNA news agency reported on Friday.

He noted "genuine peace and security of the country are guaranteed only by the strong physical force capable of defending its sovereignty," KCNA said, adding he "set forth important tasks for further increasing the strike ability".

The test of two short-range missiles on Thursday and firing of a series of projectiles on Saturday were the first launches by the North since November 2017 when it shot an intercontinental ballistic missile.

Later, Kim declared the building of its nuclear force complete and went on to hold three summit meetings with South Korean President Moon Jae-in and two with Trump.

Trump and Moon say the latest missile tests are not helpful but shouldn't scupper dialogue.

"I know they want to negotiate, they're talking about negotiating, but I don't think they're ready to negotiate," Trump told reporters.

"They were smaller missiles, they were short-range missiles."

South Korea's Moon said on Thursday the tests were likely a reaction to the failed second summit with Trump in Hanoi in February. He also thought North Korea remained hopeful of continuing negotiations.

The latest tests were swiftly followed by US test-launches of the ICBM Minuteman III over the Pacific and the Trident II submarine-launched ballistic missile off Florida.

They also coincided with a visit to the South by US special envoy for North Korea Stephen Biegun, who met his South Korean counterpart and was to hold talks with presidential Blue House and Unification Ministry officials.

The two missiles on Thursday were fired from Kusong, northwestern of Pyongyang, and flew 420km and 270km. They reached an altitude of about 50km before hitting the sea.

Washington has given no sign it will budge on the North's demand to lift some sanctions as it announced on Thursday the seizure of a North Korean cargo vessel it said was involved in the illicit shipping of coal.

The Justice Department said the 17,000-tonne Wise Honest is one of the North's largest cargo ships.