The party, known as PKK from its Kurdish language initials, was set up in 1978 to demand an independent state in southeastern Anatolia, the part of Turkey where most of the country's Kurds live.

It launched an armed struggle in August 1984; since then the conflict is believed to have cost over 37,000 lives in all.

The Kurds, one of the largest ethnic groups in the world which has never had a state of its own, are scattered across four countries: southeastern Turkey, northern Iraq, northwestern Iran and northern Syria.

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.

The proximity of the border with Iraq, and the creation there of a Kurdish administration after the first US-led war against Saddam Hussein in 1991, has provided PKK militants with a haven.

Most of the group's guerrillas are currently believed to be there, while its political leadership operates mainly out of Western capitals.

Over the years, the PKK's demands have evolved from outright independence to autonomy within a federal Turkey, plus an amnesty for its militants.

Turkey, which captured the PKK's founder and leader Abdullah Ocalan in 1999 and has since jailed him for life, has systematically refused the group's demands, and has also turned down successive offers of ceasefires proposed by the guerrillas.

In common with the United States and the European Union, Turkey views the PKK as a terrorist organisation and therefore refuses to have any contact with it.

According to the International Institute of Strategic Studies in London, the PKK currently has between 3,000 and 5,000 active guerrillas.

Although that is probably less than half of its strength during the first phase of the struggle, between 1984 and 1999, it is probable that Turkey's latest operations in Iraq will attract new recruits, according to analysts.

In recent years Turkey's actions against the PKK have been limited by the country's desire to become a member of the European Union, which has laid down a number of conditions relating to human rights and the recognition of ethnic minorities.

Ocalan, who is being held in an island prison in northwestern Turkey, still has considerable influence over the group's operations, but its main military leader is Murat Karayilan, a hardliner who is believed to be in northern Iraq.