North Korea boasted that it has become a "proud nuclear power" and threatened Monday 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 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.

On Monday, the North's main Rodong Sinmun newspaper said it is "nonsense" to say the country threatens the U.S. 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 U.S. should take a correct look at whom it is dealing with," the paper said. "It would be a grave mistake for the U.S. to think it can remain unhurt if it ignites the fuse of war on the Korean peninsula."

North Korea says its nuclear program is a deterrent against the U.S., which it routinely accuses of plotting to topple its communist regime. The U.S., which has 28,500 troops in South Korea, has repeatedly said it has no such intention and has no nuclear weapons there.

Obama said the U.S. 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. Defense Secretary Robert Gates has ordered additional protections for Hawaii in case a missile is launched over the Pacific Ocean.

"This administration — and our military is fully prepared for any contingencies," Obama said Friday during an interview with CBS News' Harry Smith, to be broadcast Monday on "The Early Show."

"I don't want to speculate on hypotheticals," Obama said. "But I want ... to give assurances to the American people that the t's are crossed and the i's are dotted in terms of what might happen."

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 from the United States and the European Union, has reportedly bought weapons from the North.

The ship is reportedly the first North Korean vessel to be tracked under the new U.N. sanctions.

Two U.S. officials said Thursday that the U.S. military had begun tracking the ship, which left a North Korean port Wednesday and was traveling off the coast of China.

One of the officials said it was uncertain what the Kang Nam was carrying, but that it had been involved in weapons proliferation before. Both spoke on condition of anonymity to discuss intelligence.

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.

McCain said Sunday that the U.S. should board the Kang Nam even without North Korean permission if hard evidence shows it is carrying missiles or other cargo in violation of U.N. resolutions.

"I think we should board it. It's going to contribute to the proliferation of weapons of mass destruction to rogue nations that pose a direct threat to the United States," he said on CBS' "Face the Nation."