Hakan Fidan, head of the National Intelligence Organisation (MIT), was summoned to testify Thursday together with two other former officials.

But the agency sent a statement to the prosecutor's office saying that under MIT rules the prime minister's office must approve any request for cooperation with an investigation, private NTV television reported.

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

Recordings of the talks were leaked in the media last year, sparking opposition ire and sending shockwaves through a public accustomed to Ankara's long-standing blanket refusal to talk to the PKK.

But the talks failed, and fighting between Turkey's military and Kurdish rebels has escalated.

In recent months, the government has also stepped up pressure on alleged rebel sympathisers.

The drive is part of a crackdown on the banned Kurdish Communities Union (KCK), the suspected political wing of the PKK. Fidan and two other MIT officials planned to testify as part of an ongoing probe into the KCK.

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.