A high-ranking North Korean official is believed to have visited China after an airliner from the North landed in the Chinese port city of Dalian.
South Korea’s Yonhap News Agency reported that North Korean leader Kim Jong Un may have been aboard the plane.
Mr Kim visited China in March for the first time since taking power six years ago, a trip that was not publicly confirmed by the sides until after he had returned to North Korea by armoured train.
Yonhap said the plane arrived on Monday amid tight security.
Japanese broadcaster NHK ran a picture of the Air Koryo plane that it said had been taken on Tuesday afternoon at Dalian’s airport.
Japan’s Kyodo News agency reported that the plane departed from Dalian later on Tuesday.
While there are no regularly scheduled flights between North Korea and Dalian, North Koreans are frequent visitors to the city and its port has been instrumental in two-way trade.
China is North Korea’s only major ally, although trade between the countries has fallen off by around 90% as Beijing enforces United Nations economic sanctions in response to the North’s nuclear bomb and ballistic missile tests.
Chinese foreign ministry spokesman Geng Shuang told reporters that he had “no information to offer at this moment”.
“China and North Korea are close neighbours and always maintain normal communication and exchanges with each other,” Mr Geng said.