Security forces have launched a manhunt for 18 prisoners allegedly associated with the Kurdistan Workers' Party (PKK) who escaped through a tunnel from a medium security facility in eastern Turkey, police officials say.

Sadullah Ergin, Turkey's justice minister, said on Wednesday that four fugitives of the outlawed group had already been convicted and the rest were being held on charges of, what he called, "membership of a terrorist organisation".

The prisoners escaped by digging a 70m long tunnel out of the jail near the city of Bingol and prompted a police operation to track them down.

Security officials have said that they are believed to be hiding in the mountainous, forested topography that surrounds the jail in Bingol.

Al Jazeera's Anita McNaught, reporting from Antakya in Turkey, said that authorities think that it may have taken the 18 inmates as long as a year to dig the tunnel, which is only 65cm in diameter.

"They haven't managed to find any of the soil from the digging nor any of the tools used to dig it, and are still searching for the 18 inmates who seem to have completely disappeared," she said.

The incident occurred just days after Recep Tayyip Erdogan, prime minister, announced that he is to unveil a "democratisation package" on September 30, which is a package of reforms designed to strengthen democracy and keep the fragile peace process with the PKK on track.

The PKK has been fighting for autonomy for the mainly Kurdish southeastern region for almost three decades in a conflict that has killed more than 40,000 people.

A peace process between Turkey and the PKK has faltered in recent months, with the PKK accusing the government of delaying rights reforms it has sought for the estimated 15 million Kurds in the country, in exchange for a ceasefire and retreating to Northern Iraq as part of a peace deal.

The PKK is designated as a "terrorist organisation" by Turkey, the European Union and US.

Source: Al Jazeera and agencies