North Korean leader Kim Jong Un attends a welcome ceremony as he arrives at the railway station in the Russian far-eastern city of Vladivostok on April 24, 2019.

The leader of North Korea ordered its military to boost its strike capability as he directed another missile firing, state media said on Friday, as tensions grew over tests that appeared to show preparations for a new advanced missile system.

The call for "full combat posture" by Kim Jong Un came while the United States announced it had seized a large 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 U.S. President Donald Trump collapsed over U.S. demands for Pyongyang's nuclear disarmament and Kim's demands for relief from punishing sanctions.

"(Kim) stressed the need to further increase the capability of the defense 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.

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 the firing of a series of projectiles on Saturday were the first missile launches by the North since November 2017 when the North shot an intercontinental ballistic missile (ICBM).

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

Both Trump and Moon said the latest missile tests were not helpful but suggested they would not 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," he said. "Nobody's happy about it but we're taking a good look and we'll see."

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