SEOUL: A North Korean ship has changed course after being tracked by the US Navy on suspicion of carrying weapons, a Pentagon official said, as Pyongyang warned Wednesday it would hit back against attempts to search its vessels.

The official declined to say where the Kang Nam 1, which left home on June 17 and was the first ship to be tracked under new UN sanctions, is now headed after it turned back.

It was reportedly originally bound for Myanmar.

The tougher sanctions, imposed in response to the North's May nuclear test, calls on UN member states to inspect cargoes if they suspect these are banned weapons shipments to or from the North.

The North responded defiantly, vowing to build more nuclear bombs.

On Wednesday it warned of military action against its arch-enemy Japan should Tokyo stop its vessels for cargo inspections.

Rodong Sinmun, daily of the ruling communist party, said Tokyo is pushing for a new law to authorise tougher cargo inspections.

"Our ships are sacred and impregnable places where our sovereignty reigns. If anyone hurts them, it would be considered a grave military provocation against us," Rodong said in a commentary.

"This kind of action will immediately meet with our self-defensive military actions and the responsibility for all consequences will rest with Japan."

Japan, along with the United States, pushed hard for tough sanctions after the North's April 5 long-range rocket launch and its second underground atomic test on May 25.

On Tuesday, Washington ordered sanctions on an Iranian-based firm for allegedly aiding the North's missile programme, and accused the two nations of joint arms proliferation.

The US Treasury named the firm as Hong Kong Electronics, located in Iran's Kish Island, and said it had been "providing support to North Korea's Tanchon Commercial Bank and Korea Mining Development Trading Corp (KOMID)."

A statement said Hong Kong Electronics has since 2007 "transferred millions of dollars of proliferation-related funds on behalf of Tanchon and KOMID" and has "facilitated the movement of money from Iran to North Korea on behalf of KOMID."

It said Tanchon has been involved in financing ballistic missile sales from KOMID to Iran's Shahid Hemmat Industrial Group, which it said was responsible for developing liquid-fuelled missiles.

"Today's action is a part of our overall effort to prevent North Korea from misusing the international financial system to advance its nuclear and missile programs and to sell dangerous technology around the world," said Stuart Levey, Treasury undersecretary for terrorism and financial intelligence.

Meanwhile, the UN's food aid agency said North Koreans, especially children, faced a "critical" food situation as donations had dried up amid the country's nuclear standoff with the world.

Torben Due, the World Food Programme's country representative in North Korea, said Pyongyang had told the agency to scale back its operations in the impoverished country without giving clear reasons.

He said the WFP, which launched an emergency operation in North Korea late last year, has had to pare back its goal of reaching 6.2 million people and is now targeting just 2.27 million.

"For adults, it doesn't mean a lot if you live for a few months on a diet of cereals and vegetables, but for children, it is critical," he told reporters in Beijing.

"We see an increase in the number of children being admitted to hospitals with severe malnutrition," Due added.

"We have not really received any contributions after the nuclear test was carried out," he said, without speculating on the reasons for the drop in donations.