An international group of journalists strongly condemned a fire-bomb attack on the Nihon Keizai Shimbun's headquarters in Tokyo on Friday.

No arrests have been made, but the attack, which caused minor damage and no injuries, bears all the hallmarks of extreme rightwing fringe groups, Paris-based Reporters Without Borders said in a statement.

The bombing appears linked to a recent exclusive story in the Nikkei, based on the private diaries of a close aide to former Emperor Hirohito, which said that from 1978 he refused to visit Yasukuni Shrine after 14 war criminals were enshrined there.

"Nikkei's courageous revelations have infuriated ultranationalist groups," the press freedom organization said. "Ultranationalists have launched previous attacks on the press, murdering a journalist from the Asahi Shimbun in 1987.

"We condemn this attack as well as the threats and harassment against journalists accused of sullying Japan's Imperial past," it said. "It is essential that the government ensures the safety of Nikkei and identifies those who carried out this attack. We will keep a close eye on the police investigation."