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.
A senior U.S. defense official, speaking on condition of anonymity because he was unable to discuss intelligence on the vessel, 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.
North Korea has said it would consider interception 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.