Photographs of journalists, (R-L) Orhan Dzhemal, Kirill Radchenko and Alexander Rastorguyev, who were recently killed in Central African Republic by unidentified assailants, are on display outside the Central House of Journalists in Moscow. File picture: Maxim Shemetov/Reuters

Moscow - Russian tycoon Mikhail Khodorkovsky has pulled funding for an investigative journalism project, saying it made mistakes organising a trip to Central African Republic where three of its reporters were killed investigating Russian mercenaries.

The three reporters, working for the TsUR online news outlet, travelled to CAR to investigate the activities of clandestine Russian private military contractors known as the Wagner Group when they were ambushed and killed on July 30.

Khodorkovsky, who has vowed to find those responsible, said the choice of local assistant for the reporters had been a "significant mistake" and that responsibility for that lay in part with TsUR's producers.

Khodorkovsky said he had received a letter of resignation from Andrei Konyakhin, TsUR's chief editor.

"Even partial responsibility is too much in a situation like this. I therefore note the resignation letter of TsUR's chief editor Andrei Konyakhin and stop my support of the (TsUR) project as an investor," he wrote on a social media post.

Russian officials have said a preliminary investigation into the murder of the journalists showed they had been killed by assailants who wanted to rob them, but some Russian journalists have said the circumstances and murder motive remain unclear.

Khodorkovsky has set up a foundation to investigate the murder and is offering a financial reward for information about the crime, he said.

"We have reason to believe the group were the victims of a planned operation," Khodorkovsky said, without saying on what evidence that belief was based.