North Korea boasted that it has become a "proud nuclear power" and threatened today to harm the US if attacked, as tensions mounted over a possible crackdown on exports of suspected missile parts from the North.
President Barack Obama said the US is ready to cope with "any contingencies" involving North Korea and vowed not to "reward belligerence and provocation".
South Korea's YTN news network reported that a US Navy destroyer was tailing a North Korean ship suspected of carrying missiles and related parts toward Burma in what could be the first test of new UN sanctions against the North over its recent nuclear test.
The sanctions toughen an earlier arms embargo against North Korea and authorise ship searches in an attempt to thwart its nuclear and ballistic missile programs.
Tensions on the Korean peninsula have spiked since North Korea defiantly conducted its second nuclear explosion on May 25.
It later declared it would expand its atomic bomb programme and threatened war to protest against the UN sanctions imposed in response to its nuclear test.
Today, the North's main Rodong Sinmun newspaper said it is "nonsense" to say the country threatens the US and claimed it is Washington that threatens the North. The paper also warned that the country is prepared to strike back if attacked.
"As long as our country has become a proud nuclear power, the US should take a correct look at whom it is dealing with," the paper said. "It would be a grave mistake for the US to think it can remain unhurt if it ignites the fuse of war on the Korean peninsula."
North Korea says its nuclear programme is a deterrent against the US, which it routinely accuses of plotting to topple its communist regime. The US, which has 28,500 troops in South Korea, has repeatedly said it has no such intention and has no nuclear weapons there.
Mr Obama said the US is prepared for any North Korean provocation, including the regime's reported threat to test-launch a long-range missile toward Hawaii.
Japanese media have reported that the North Koreans appear to be preparing for a long-range test timed for about July 4. Defence Secretary Robert Gates has ordered additional protections for Hawaii in case a missile is launched over the Pacific Ocean.