The hardline communist state, angrily rejecting what it calls US-inspired sanctions imposed for its May 25 nuclear test, promised Tuesday to retaliate against them.
In Beijing, US and Chinese defence officials were to hold talks later in the day with the Americans expected to seek support for the UN sanctions.
In the first move to enforce the measures, a US destroyer was tracking a North Korean ship with a suspected weapons cargo. A US defence official said it was believed headed for military-ruled Myanmar.
Japan's Coast Guard said the North had warned that military drills would be carried out off its eastern port of Wonsan between Thursday and July 10.
The alert was seen as signalling possible short-range or medium-range missile tests. The North is also thought to be preparing a long-range missile launch from another location into the Pacific, short of Hawaii.
Washington has said it is prepared for the possibility that North Korea could fire a missile towards Hawaii, perhaps on the July 4 US Independence Day -- a scenario reportedly outlined in a Japanese defence ministry paper.
Pyongyang's April 5 rocket launch sparked off the latest standoff. Angry at UN condemnation of the exercise, it quit international nuclear disarmament talks and staged its second atomic test -- attracting further sanctions.
In the days after the test the North fired a series of short-range missiles off its east coast and renounced the truce in force on the peninsula, prompting South Korea to reinforce its military on the tense border.
Pyongyang's cabinet newspaper Minju Joson Tuesday accused Washington of a naval and air buildup around the Korean peninsula to try to browbeat it.
The paper said sanctions and US pressure would never work on the North, which would "resolutely counter the US 'sanctions' with retaliation and 'confrontation' with all-out confrontation."
The new UN sanctions approved June 12 ban arms shipments -- including missile-related cargo -- to and from the North. But the resolution rules out military force to enforce the measures.
A US defence official told AFP the Kang Nam 1, which is now being tracked, appeared headed for Myanmar with which the North has friendly ties.
US officials have yet to indicate if or when they might ask to search the vessel.
The North Koreans are expected to reject any such request. But at some point the ship will likely need to stop for refuelling, possibly in Singapore where authorities have the right to search it.
China, North Korea's sole major ally and biggest trade partner, is seen as crucial to effective enforcement of the UN measures designed to cripple the North's atomic and long-range missile programmes.
North Korea was set to be a major topic in the talks between Chinese officials and a US delegation led by Michele Flournoy, under-secretary for defence.
A Pentagon official told reporters in Washington over the weekend that Pyongyang's nuclear test and missile launches have caused "great concern" in Beijing.
"We would hope that China would use whatever influence they have with North Korea to convince them to change their behaviour," he added.