The ship is the first vessel to be monitored under new UN sanctions designed to enforce a ban on North Korea from selling arms and nuclear technology.
North Korea's weapons trade has long been a vital source of foreign income for the impoverished and isolated communist country.
But its weapons exports, carried out behind a cloak of secrecy, are likely to become increasingly difficult as the US cracks down on companies helping the trade.
The US moves and new UN sanctions come in response to North Korea's second nuclear test conducted in May.
Under the new sanctions, air, sea and land shipments out of North Korea can be subject to boarding and inspection, although North Korea has warned that any moves to do so will be seen as a declaration of war.
A US official, speaking on condition of anonymity, told Reuters late on Tuesday the Kang Nam appeared to be heading back in the direction of North Korea after turning around within the last few days.
"We've no idea where it's going," the official said. "The US didn't do anything to make it turn around."
The ship was suspected of carrying missile parts and had been headed toward Myanmar, South Korean media had reported.
It left port in North Korea several days ago and has been tracked by the US navy along the coast of China.
On Tuesday it was believed to be located in seas south of Hong Kong.
North Korea and Myanmar have drawn closer in recent years, perhaps deepening their affinity as both face escalating international sanctions.
On Monday Japanese police arrested three people, including one North Korean resident of Japan, on suspicion of trying to export to Myanmar a magnetic measuring device that could be used in missile construction, the Yomiuri newspaper said.
Later this week a top US envoy is due to arrive in Beijing to seek Chinas help in stepping up sanctions against North Korea's weapons programme.
Philip Goldberg, the US envoy for coordinating arms and other sanctions against North Korea under the recent UN resolution, will hold talks with senior Chinese officials.
China is North Korea's closest ally but backed the UN resolution condemning the North's nuclear test and imposing fresh sanctions on its arms trade.