A group supporting late Russian opposition leader Alexei Navalny says Russian authorities have informed his family that Navalny's body will not be handed over another 14 days.

The authorities on Friday announced Navalny's death in a prison where he was serving a jail sentence. He was a fierce critic of President Vladimir Putin.

A spokesperson for his support group said in a social media post on Monday that the authorities said the official probe into Navalny's death had been extended.

The bereaved family has been denied access to his body and details into the cause of his death.

Russian presidential spokesperson Dmitry Peskov said that the Kremlin was not involved in the proceedings regarding the handover of Navalny's body to his family. He added that it's not a job for the administration.

More than 60,000 appeals have been sent to a Russian human rights group, demanding that the country's Investigative Committee hand over Navalny's body to his family.

Navalny's widow, Yulia Navalnaya, said in a video message on Monday that Vladimir Putin killed her husband. She said the Kremlin refuses to hand over his body because they are waiting for traces of poison to disappear from the body.

Navalnaya added that she will continue her husband's work and continue the struggle for her country.