The sound of an explosion could be heard on the black boxes recovered from a Russian plane that crashed over Egypt's Sinai Peninsula on Saturday, according to an investigator who had access to them, French TV station France 2 said on its website on Friday.

According to the investigator, the explosion was not consistent with an engine failure, the report said.

Earlier, Russian President Vladimir Putin halted all Russian flights to Egypt amid growing evidence a bomb downed a St Petersburg-bound charter plane that crashed killing all 224 people aboard.