Istanbul's deputy prosecutor Fikret Secen said Sadrettin Sarikaya was off the investigation into the KCK, an underground group thought to have links to the banned Kurdistan Workers Party (PKK), Anatolia news agency reported.

He would be replaced by two other magistrates, Secen said.

The move against Sarikaya came after he summoned Hakan Fidan, the head of the National Intelligence Organisation (MIT), for questioning, Anatolia reported.

He also issued arrest warrants against former MIT chief Emre Taner, another former senior officer and two active members of the service after they ignored a summons for questioning on Thursday.

Turkish media reported that the MIT officials had been summoned by the prosecutor because, according to him, they had exceeded their powers by seeking a settlement with the outlawed organisation during the negotiations.

But the MIT had sent a statement to the Istanbul prosecutor to say that regulations required the prime minister's authorisation for any investigation into the intelligence service, said media reports.

Fidan, who was appointed by Prime Minister Recep Tayyip Erdogan, had secret talks with representatives of the PKK in 2010 in Oslo.

Recordings of the Oslo talks between MIT and the Kurdish rebels were leaked in the media last year, sparking outrage among opposition politicians.

They accused the ruling Justice and Development Party (AKP) of having abandoned the official position of no contact with the PKK.

The PKK took up arms in southeastern Turkey in 1984, sparking a conflict that has claimed some 45,000 lives. It is labelled a terrorist outfit by Ankara and much of the international community.

But many specialists argue that there is no military solution to the conflict and that political talks are required.

The investigation into the KCK has so far led to the arrest of hundreds of Kurds and was one fact in the failure in 2009 of a more conciliatory government policy towards the Kurdish community.