The United States Defence Secretary says the US is preparing for a possible missile attack by North Korea on Hawaii.
Robert Gates says he's ordered missile interceptors and a radar system to be deployed to the islands - the site of the main American military base in the Pacific.
"We do have some concerns if they were to launch a missile to the West, in the direction of Hawaii," he says.
State Department spokesperson P J Crowley says there are concerns that North Korea might retaliate after the United Nations imposed tough new sanctions on it for testing a nuclear weapon last month.
Suspect ship's movements being tracked
The Pentagon is also tracking the movement of a North Korean ship to ensure it does not break UN sanctions against exporting arms.
Officials have declined to say what the ship, the Kang Nam, might be carrying but say it has become a "subject of interest".
"North Korea will endlessly try to export arms," says Cho Myung-chul, an expert on the North's economy at South Korea's Institute for International Economic Policy.
"They are unable to shake this thought off their minds, because exporting arms is a very profitable business compared to other goods."