Major South Korean newspapers splashed a photo of Japanese Prime Minister Shinzo Abe in a military jet trainer on their front pages Wednesday, saying it was a reminder of Japan's colonial-era atrocities.
The picture in question showed a smiling Abe giving a thumbs-up while sitting in the cockpit of an air force T-4 training jet emblazoned with the number 731.
The number evoked memories of Unit 731 -- a covert Japanese biological and chemical warfare research facility that carried out lethal human experiments during the 1937-45 Sino-Japanese War and World War II.
The unit was based in the northeastern Chinese city of Harbin, and held prisoners from China, South Korea and the Soviet Union.
The press in Seoul suggested the Abe picture was an intended affront to countries like China and South Korea which had suffered under Japanese occupation and colonisation.
"Abe's endless provocation!" said the picture caption on the front page of the country's largest daily, the Chosun Ilbo.
"Abe's pose resurrects horrors of Unit 731," ran the headline in the English-language Korea JoongAng Daily.
The picture was reportedly taken on Sunday at an air force base in Japan's Miyagi Prefecture. Abe was visiting the base as part of a tour of areas affected by the 2011 tsunami.
The Japanese Defence Ministry suggested the number on the trainer was simply coincidental.
"There was no particular meaning in the number of the training airplane the prime minister was in on Sunday. Other than that there is nothing we can say," a ministry spokesman told AFP in Tokyo.
The prominence given to the photo will likely fuel public anger in South Korea, which has already been aroused by the recent visit of Japanese cabinet ministers and lawmakers to a controversial war shrine.
The Yasukuni shrine in central Tokyo honours 2.5 million war dead, including 14 leading war criminals and is regarded by South Korea and China as a symbol of wartime aggression.
South Korean Foreign Minister Yun Byung-Se shelved a proposed trip to Tokyo in protest at the visits, while President Park Geun-Hye warned Japan against shifting to the right and aggravating the "scars of the past".