WASHINGTON -- Former Trump administration national security adviser Michael Flynn is not ready to be sentenced yet, special counsel Robert Mueller said Friday in a one-page filing that suggests the former White House official is still co-operating with prosecutors.

Flynn pleaded guilty in December to lying to the FBI about whether he discussed sanctions during the transition period with the Russian ambassador at the time. He has been co-operating with Mueller's investigation into possible co-ordination between Russia and the Trump campaign.

He has been awaiting sentencing, with guidelines calling for a range between zero to 6 months in prison.

But prosecutors with Mueller told a judge in a court filing Friday that they were not ready to set a sentencing hearing. The filing, done jointly with lawyers for Flynn, asks for permission to submit another status report by August 24.

That suggests prosecutors are continuing to work with Flynn to get information for their investigation.

The status stands in contrast to the case of George Papadopoulos, a former Trump campaign adviser who also admitted lying to the FBI. He has been co-operating with Mueller as well, and prosecutors recently set a September sentencing date.

Another former Trump aide, Rick Gates, pleaded guilty in February and agreed to co-operate.