The move was aimed at addressing Council of Europe criticism that Ankara was violating Ocalan's human rights by keeping him in solitary confinement on Imrali island in the Sea of Marmara, northwestern Turkey.

"There is a total of six inmates in Imrali as of this morning," Justice Minister Sadullah Ergin told reporters in Istanbul.

"This was made in order to overcome the criticism that we have faced, to prevent Turkey from being unfairly criticised," he added.

The minister said four of the men transferred to the island were convicted members of Ocalan's outlawed Kurdistan Workers' Party (PKK), which has fought Ankara since 1984, while one belonged to an underground far-left group.

Under Turkish law, Ocalan, 61, will be now able to socialise with them in recreational facilities for up to 10 hours per week.

In preparation for the newcomers, Turkey has built additional facilities at the prison, which can now accommodate a total of nine inmates.

After several visits to Imrali, the Council of Europe's Committee for the Prevention of Torture and Inhuman or Degrading Treatment found the material conditions in Ocalan's cell to be satisfactory, but slammed Ankara for keeping him in solitary confinement and restricting his contacts with the outside world.

Ocalan, founder and leader of the PKK, was condemned to death for treason and separatism in 1999 following his capture in Kenya, where he was on the run after leaving his long-time safe haven in Syria.

The sentence was commuted to life imprisonment in 2001 following Turkey's abolition of capital punishment as part of reforms to embrace European Union norms.

The PKK, listed as a terrorist group by Turkey and much of the international community, took up arms in the mainly-Kurdish southeast in 1984, sparking a conflict that has claimed about 45,000 lives.