The group, which has Marxist-Leninist roots, was created in 1978 to demand an independent Kurdish state in southeastern Anatolia, the part of Turkey where most of the country's Kurds live.

It launched an armed struggle in August 1984 and since then the conflict has killed around 45,000 people.

The group is considered a terrorist organisation by Turkey, the United States and the European Union.

The PKK suffered a major blow in 1999 when its founder and leader Abdullah Ocalan was arrested and condemned to death for treason. The sentence was commuted to life in prison in 2002 after Turkey dropped the death penalty.

Turkey is believed to be home to the largest single community of ethnic Kurds -- between 13 and 19 million, out of a total population of between 25 and 35 million scattered across Iraq, Iran and Syria.

The proximity of the border with Iraq, and the creation there of a Kurdish administration after the Gulf War in 1991, has provided PKK militants with a haven.

The group is believed to have its command base there, while the political leadership operates mainly out of European capitals.

Over the years, the PKK's demands have evolved from outright independence to autonomy as well as cultural and language rights.

It also developed an urban wing, the Kurdistan Communities Union (KCK), consisting of mayors and other local officials to campaign for Kurdish political rights.

Thousands of KCK members remain behind bars after a national crackdown launched in 2009.

Between 2009 and 2011, secret talks took place between representatives of the Turkish government and members of the PKK in Norway. But the talks were deadlocked and collapsed after Turkish soldiers were killed in June 2011 following clashes between the PKK and the army.

Late last year, Turkish intelligence officials resumed talks aimed at ending the conflict, holding several meetings with Ocalan in his remote island prison cell.

Turkey, under pressure from the EU which it aspires to join, is currently also working on a new constitution granting Kurds increased rights.

The PKK is believed to have between 3,000 and 5,000 active guerrillas -- possibly less than half of its strength until 1999.