The UK has frozen the assets of six Russian prison bosses in charge of the Arctic penal colony where opposition leader Alexei Navalny died.
The sanctioned individuals will also be banned from travelling to the UK.
Western leaders say the blame for Navalny's death lies with the Russian authorities, including President Putin.
"Those responsible for Navalny's brutal treatment should be under no illusion - we will hold them accountable," UK Foreign Secretary Lord Cameron said.
The British government has also called for Navalny's body to be released to his family immediately and for a full and transparent investigation to take place.
The appeal from the UK echoes similar calls from Navalny's mother, who was filmed on Tuesday outside the colony where he died saying she had been trying to see him for five days but did not even know where he was.
Those sanctioned by the UK are:
- Col Vadim Konstantinovich Kalinin - head of the penal colony
- Lt Col Sergey Nikolaevich Korzhov - deputy head
- Lt Col Vasily Alexandrovich Vydrin - deputy head
- Lt Col Vladimir Ivanovich Pilipchik - deputy head
- Lt Col Aleksandr Vladimirovich Golyakov - deputy head
- Col Aleksandr Valerievich Obraztsov - deputy head
