North Korean leader Kim Jong-un has ordered his military to boost its strike capability as he directed another missile firing, state media said.
The call for "full combat posture" by Mr Kim 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 Mr Kim and US President Donald Trump collapsed over US demands for Pyongyang's nuclear disarmament and Mr Kim's demands for relief from punishing sanctions.
"(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.
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 yesterday 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, Mr 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 Mr Trump.
Both Mr Trump and Mr 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," Mr 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."
President Moon said the tests were likely a reaction to the failed second summit with Mr Trump in Hanoi in February.
The latest tests were swiftly followed by US test-launches of the intercontinental ballistic missile Minuteman III over the Pacific and the Trident II submarine-launched ballistic missile (SLBM) off Florida.
They also coincided with a visit to the South by US special envoy for North Korea Stephen Biegun, who was scheduled to hold talks with his South Korean counterpart and with presidential Blue House and Unification Ministry officials.
The two missiles yesterday were fired from Kusong, an area northwest of Pyongyang and flew 420km and 270km and reached an altitude of about 50km before hitting the sea, South Korea's military said.
North Korea's state media did not provide details of the missiles.
The South Korean and US militaries are analysing the tests, which will include whether they were the Iskander short-range ballistic missile system developed by Russia.
Analysts said the multiple tests signalled that North Korea is serious about deploying an operational fleet of the missiles, which could be used in the opening hours of any war with the United States and South Korea.
Washington has given no sign it will budge on the North's demand to lift some sanctions as it announced the seizure of a North Korean cargo vessel that it said was involved in the illicit shipping of coal.
The Justice Department said the 17,061-tonne Wise Honest is one of the North's largest cargo ships and it was first detained by Indonesia in April 2018 but is now in the possession of the United States.
The announcement has no connection with the North's missile activities, a US official said.