SEOUL, South Korea — North Korea boasted that it has become a "proud nuclear power" and threatened today to harm the U.S. if attacked, as tensions mounted over a possible crackdown on exports of suspected missile parts from the North.

President Obama said the U.S. is ready to cope with "any contingencies" involving North Korea, including the regime's reported threat to test-launch a long-range missile toward Hawai'i, and vowed not to "reward belligerence and provocation."

South Korea's YTN news network reported that a U.S. Navy destroyer was tailing a North Korean ship suspected of carrying missiles and related parts toward Myanmar in what could be the first test of new U.N. sanctions against the North over its recent nuclear test.

The sanctions toughen an earlier arms embargo against North Korea and authorize 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 program and threatened war to protest the U.N. sanctions imposed in response to its nuclear test.

Japanese media have reported that the North Koreans appear to be preparing for a long-range test timed for about July 4. Defense Secretary Robert Gates has ordered additional protections for Hawai'i in case a missile is launched over the Pacific Ocean.

The North's cargo ship, Kang Nam, is expected to travel to Myanmar via Singapore, YTN said, citing an unidentified intelligence source in the South. Myanmar's military government, which faces an arms embargo by the United States and the European Union, reportedly has bought weapons from the North.

A senior U.S. military official told The Associated Press on Friday that a Navy ship, the USS John S. McCain, is relatively close to the North Korean vessel but had no orders to intercept it under the U.N. Security Council resolution and had not requested that authority. The official spoke on condition of anonymity because of the sensitivity of the issue.

The Navy ship, a guided-missile destroyer, is named after the grandfather and father of former U.S. presidential candidate Sen. John McCain. Both were admirals.