SEOUL, South Korea - A North Korean ship monitored for more than a week by the U.S. Navy has changed course and is heading back the way it came, U.S. officials said, as Pyongyang warned Wednesday it will take military action if anyone attempts to search its vessels.

The Kang Nam 1 -- originally believed to be bound for Myanmar with suspicious cargo on board, possibly illicit weapons -- turned around and headed back north on Sunday, two U.S. officials said on condition of anonymity to discuss intelligence.

The U.S. officials, speaking in Washington on Tuesday, said they do not know where the ship is going. But it was some 400 kilometres south of Hong Kong on Tuesday and heading north, one official said.

The North Korean ship is the first vessel monitored under UN sanctions aimed at punishing the regime for conducting an underground nuclear test in May.

The new resolution seeks to clamp down on North Korea's trading of banned arms and weapons-related material by requiring UN member states to request inspections of ships suspected of carrying prohibited cargo.

The communist nation has said it would consider interception of its ships a declaration of war. On Wednesday, North Korea's main Rodong Sinmun newspaper renewed the warning.

"Touching our ships constitutes a grave military provocation against our country," the paper said in commentary carried by the official Korean Central News Agency. "These acts will be followed immediately by self-defensive military countermeasures."

The North's warning did not specifically mention the Kang Nam 1, which the two U.S. officials said has been moving very slowly in recent days in a possible sign it was trying to conserve fuel. The resolution prohibits UN members from providing fuel to ships suspected of carrying banned items.

The officials said they did not know what the ship's turnaround means, nor what prompted it.

Myanmar's authorities had informed the North Korean ambassador that it would not allow the Kang Nam to dock if it was carrying weapons or other banned materials, a Radio Free Asia report said.

Koh Yu-hwan, an expert at Seoul's Dongguk University, said North Korea appears to have decided to bring home its vessel because of Myanmar's reaction.

"The North's cargo ship appears to have changed its course as the country's rogue image could be further strengthened if illegal weapons were on board," he said.

A U.S. delegation headed by envoy Philip Goldberg, meanwhile, arrived in Beijing on Wednesday to discuss the UN sanctions with Chinese officials later this week. Goldberg, a former ambassador who is in charge of coordinating the sanctions' implementation, did not speak to reporters at the airport.

China's cooperation in enforcing the sanctions against neighboring North Korea, which counts Beijing as its main ally, is seen as crucial to encouraging the North back to nuclear disarmament talks the regime abandoned in April.

Pyongyang also threatened in April to launch a long-range missile. A no-sail zone remains in effect off North Korea's east coast through July 10. An announcement cited "military drills" but there were concerns the defiant nation might test-fire short- or medium-range missiles, or even a long-range missile, in further violation of Security Council resolutions.

However, there was no sign of an imminent missile launch Wednesday, an official at South Korea's Joint Chief of Staff said. He asked not to be named, citing agency policy.

In Washington, the U.S. Treasury Department imposed financial sanctions on Hong Kong Electronics, a company located in Kish Island, Iran, accused of involvement in North Korea's missile proliferation network.

That means any bank accounts or other financial assets found in the U.S. belonging to the company must be frozen. Americans also are prohibited from doing business with the firm.

Meanwhile, North Korea's government has sought to whip up anti-American sentiment with a series of state-organized rallies. KCNA said Wednesday that the latest anti-U.S. demonstrations were held through Tuesday in three provinces where participants condemned the UN resolution and what the regime calls a U.S. plot to invade the country.

Such rallies have been held since June 25, the anniversary of the 1950 outbreak of the Korean War where the U.S. fought alongside South Korea against invading troops from North Korea. The war ended in 1953 in a truce, not a peace treaty, leaving the two Koreas still technically at war.

In Beijing, the UN World Food Program said Wednesday it was unable to reach millions of hungry women and children in the North due to a lack of international funding, and the North's new restrictions on its staff and where it can operate.