North Korea accused Washington of seeking to "provoke a second Korean War" as the regime prepared to hold maritime military exercises off the eastern coast.
U.S. and regional authorities were watching closely for signs that North Korea might fire short- or mid-range missiles during the June 25 to July 10 timeframe cited in a no-sail ban for military drills sent to Japan's Coast Guard.
North Korea had warned previously it would fire a long-range missile as a response to U.N. Security Council condemnation of an April rocket launch seen as a cover for its ballistic missile technology.
An underground nuclear test last month drew more Security Council action: A resolution seeking to clamp down on North Korea's trading of banned arms and weapons-related material by requiring U.N. member states to request inspections of ships carrying suspected cargo.
In a first test of the new resolution, a North Korean ship suspected of transporting illicit weapons was sailing off China's coast with a U.S. destroyer close behind.
The Kang Nam, which left the North Korean port of Nampo a week ago, is believed bound for Myanmar, South Korean and U.S. officials said.
Ship's destination, cargo still unknown
Myanmar state television downplayed the reports of a possible weapons shipment Wednesday evening, saying another North Korean vessel was expected to pick up a load of rice but that the government had no information about the Kang Nam.
A senior U.S. defense official said Wednesday that the ship had already cleared the Taiwan Strait.
He said he didn't know how much range the Kang Nam has — that is, whether or when it may need to stop in some port to refuel — but that the Kang Nam has in the past stopped in Hong Kong's port.
Another U.S. defense official said he tended to doubt reports that the Kang Nam was carrying nuclear-related equipment, saying the information officials have received seems to indicate the cargo is conventional munitions.
The U.S. officials spoke on condition of anonymity because they were discussing intelligence.
Officials said last week that they believed the ship was carrying smaller arms, though they didn't elaborate.
North Korea threatens to declare war
North Korea has said it would consider interception of its ships a declaration of war, and on Wednesday accused the U.S. of seeking to start another Korean War.
"If the U.S. imperialists start another war, the army and people of Korea will ... wipe out the aggressors on the globe once and for all," a dispatch from the official Korean Central News Agency said.
The warning came on the eve of the 59th anniversary of the outbreak of the Korean War. The brutal fighting ended after three years in a truce in 1953, not a peace treaty, leaving the peninsula divided and in a state of war. The U.S. has 28,500 troops in South Korea to protect against an outbreak of hostilities.
On Wednesday, the top U.S. commander in South Korea, Gen. Walter Sharp, praised soldiers from U.S.-led U.N. forces who died fighting the "tyranny" of communist North Korea decades ago.
"A North Korean victory in the Korean War would have brought the nightmare of tyranny to this great land, thrusting the citizens of the Republic of Korea into a darkness that their northern counterparts have yet to emerge from," he said a commemoration ceremony Wednesday, referring to South Korea by its official name, the Republic of Korea.
Reports about possible missile launches from the North highlighted the state of tension on the Korean peninsula.
No-sail ban linked to missles
A senior South Korean government official said the no-sail ban is believed connected to North Korean plans to fire short- or mid-range missiles. He spoke on condition of anonymity, citing department policy.
Yonhap reported that the North may fire a Scud missile with a range of up to 310 miles or a short-range ground-to-ship missile with a range of 100 miles during the no-sail period.
U.S. defense and counterproliferation officials in Washington said they also expected the North to launch short- to medium-range missiles. The officials spoke on condition of anonymity to discuss sensitive intelligence.
South Korea will expedite the introduction of high-tech unmanned aerial surveillance systems and "bunker-buster" bombs in response to North Korea's provocations, the Chosun Ilbo newspaper said, citing lawmakers.
Meanwhile, a flurry of diplomatic efforts were under way to try getting North Korea to return to disarmament talks.
Russia's top nuclear envoy, Alexei Borodavkin, said after meeting with his South Korean counterpart that Moscow is open to other formats for discussion since Pyongyang has pulled out of formal six-nation negotiations.
In Beijing, top U.S. and Chinese defense officials also discussed North Korea. U.S. Defense Undersecretary Michele Flournoy was heading next to Tokyo and Seoul for talks.
South Korea has proposed high-level "consultations" to discuss North Korea with the U.S., Russia, China and Japan.