BEIJING (Reuters) - A North Korean Air Koryo airplane made an emergency landing in China's northeastern city of Shenyang on Friday after smoke appeared in the cabin, but there were no injuries, airport authorities and the official Xinhua news agency said.

The plane, belonging to the North's national carrier, was flying to Beijing from Pyongyang when it made a forced landing because of the smoke, the airport said in a short statement on its microblog.

The aircraft made a safe emergency landing, and "nothing abnormal" was found in its condition, although an investigation was underway, it added.

Xinhua, which had initially reported a fire, cited a passenger as saying the smoke appeared about 30 minutes after takeoff, and flight crew told passengers not to panic as the plane landed.

"Later, oxygen masks dropped and several passengers began to have breathing difficulty because of the oxygen shortage in the cabin," Xinhua said, adding that the plane landed 10 minutes later and no one on board was injured.

Fire trucks were dispatched and the aircraft was "smoking" when ground staff examined it, but no obvious fire was spotted, the passenger told the agency, which added that rain in Shenyang at the time of the landing may have averted a fire.

An airline staff member at Pyongyang airport told Xinhua the plane had suffered a "malfunction," but more investigations were needed to determine what happened.

An Air Koryo employee reached at the airline's Shanghai office told Reuters no company representatives were available, and she was unaware of the situation. The Beijing office could not be reached.

Independent ratings website Skytrax lists Air Koryo as the world's only 1-star airline for poor quality standards, though it does not measure safety.

Few North Koreans are allowed to travel outside their isolated country.

The North Korean state-owned airline uses mainly Russian-built Tupolev aircraft on international flights but older, Soviet-era aircraft are used within the country.

The aircraft, operating flight JS 151, was a Tupolev TU204-300, plane tracking website flightradar24.com showed.

(Reporting by Michael Martina; Additional reporting by Ben Blanchard, and James Pearson in Seoul; Editing by Michael Perry and Clarence Fernandez)