A Russian investigative journalist who covered the deaths of mercenaries in Syria has died after falling from his fifth-floor apartment.

News website Novy Den confirmed its reporter Maxim Borodin had been found badly injured by neighbours in Yekaterinburg, Russia, before passing away in hospital on Sunday local time.

The cause of the fall in unclear as local officials say the incident was unlikely to be of a criminal nature.

Russia news reports citing police say the apartment was locked from the inside, however BBC reports a friend close to Mr Borodin as saying his flat had been surrounded by security men a day earlier.

Vyacheslav Bashkov said Mr Borodin had contacted him at 5am on April 11, saying there was “someone with a weapon on his balcony and people in camouflage and masks on the staircase landing”.

Mr Borodin, who had been looking to hire a lawyer, later called him back to say he was wrong and that the security were a part of an exercise.

Mr Borodin was described as a “principled, honest journalist”. No suicide not was found at the scene according to local officials.

In March, Mr Borodin wrote that thousands of Russian mercenaries had been deployed to Syria by a contracting company, Wagner, which is reportedly funded by a St Petersburg entrepreneur Yevgeny Prigozhin.

Mr Prigozhin was dubbed “Putin’s chef” by Russian media outlets as his restaurants previously hosted the Russian leader’s dinners with foreign dignitaries.

Mr Prigozhin was indicted by the US in February on charges that he funded the “troll factory” alleged to have tried to influence the 2016 presidential election.

Meanwhile in February, Mr Borodin reported as many as 200 Wagner fighters killed in battle with US soldiers in Syria. Only a dozen deaths were confirmed by Russian authorities, however CIA director Mike Pompeo said that “a few hundred” Russians were killed.

He also alleged bodies of those Russian mercenaries being delivered to a regional village.

Novy Den chief editor Polina Rumyantseva said Mr Borodin’s death may not have been an accident or suicide. 

“In order to understand what happened, we have to obtain access to the apartment, we’re working on that. If there is the slightest indication of foul play, we’ll inform everyone,” local media quoted her saying.

-with agencies