US officials suspect the Kang Nam is carrying missiles and parts to Myanmar [Reuters/File photo]

SEOUL, South Korea — A North Korean cargo ship shadowed by an American Navy destroyer was reportedly steaming toward Myanmar on Sunday, posing what could be the first test of how far the United States and its allies will go under a new United Nations resolution to stop the North’s military shipments.

The United States began tracking the ship, the 2,000-ton freighter Kang Nam, after it left Nampo, a port near Pyongyang, the capital of North Korea, on Wednesday. Pentagon officials have said they suspect the ship is carrying prohibited materials, but they have declined to say where it might be headed.

North Korea has said it would consider interception an “act of war” and react accordingly.

Over the weekend, the North’s state-run news media vowed to “respond to sanctions with retaliation” and threatened “unlimited retaliatory strikes” against South Korea if it cooperated with the sanctions imposed by the United Nations Security Council.

A South Korean cable news network, YTN, quoted an unidentified intelligence source on Sunday as saying that American authorities suspected that the ship was carrying missiles or related parts.

The network also said the Kang Nam was headed for Myanmar, formerly known as Burma, which is ruled by a military junta and has long been suspected of buying North Korean arms, as well as providing transit services for North Korean vessels engaged in illicit trade. The Kang Nam is the first North Korean vessel to be tracked under the resolution adopted by the Security Council on June 12 to punish North Korea for its May 25 nuclear test.

The resolution forbids North Korea to traffic in a wide range of nuclear and conventional weaponry and calls on United Nations members to search North Korean ships, with the crews’ consent, if there are “reasonable grounds” to suspect that banned cargo is aboard. If the crew does not accept inspection on the high seas, North Korea is to direct the vessel to a port for inspection by local authorities there.

Because Myanmar is another nation defying international weapons sanctions, a port there would be unlikely to comply with the resolution.

Shortly after North Korea conducted its first nuclear test in 2006, the Kang Nam was detained in Hong Kong following a Council resolution banning trade in nuclear and ballistic missile technology. But then the ship was found to be carrying no cargo.

The potential high-seas confrontation over the Kang Nam came as American defense officials planned to travel this week to South Korea, Japan and China to discuss how to enforce the sanctions.