UK Foreign minister David Cameron said Wednesday that London had imposed sanctions on six officials at the head of the Siberian penal colony where Russian opposition figure Alexei Navalny died.

"Those responsible for Navalny's brutal treatment should be under no illusion -- we will hold them accountable," Cameron said.

The sanctioned individuals will be subject to freezes of any UK assets and travel bans.

They include Vadim Konstantinovich Kalinin, who oversaw the IK-3 Arctic Penal Colony "Polar Wolf", and five of his deputies.

The UK, which is the first nation to issue sanctions related to Navalny's death on Friday, also called for his body to be released to his family immediately.

"It's clear that the Russian authorities saw Navalny as a threat and they tried repeatedly to silence him," Cameron said.

Russian authorities "poisoned him with Novichok in 2020, they imprisoned him for peaceful political activities, and they sent him to an Arctic penal colony. No one should doubt the oppressive nature of the Russian system," he added.

"That's why we're today sanctioning the most senior prison officials responsible for his custody in the penal colony where he spent his final months."