Why is Turkey bombarding Kurdish militants in Syria?

Turkey is warning of a possible ground operation attacking Kurdish groups in what Ankara says is retaliation for a lethal bombing in a busy Istanbul street.

The explosion on November 13 killed six people and injured more than 80, with Ankara blaming Kurdish groups for the deadly deed.

The Kurds have denied responsibility but Turkey has been bombarding areas of Iraq and Syria, which are home to the suspected groups, from the air.

“While we press ahead with air raids uninterrupted, we will crack down on terrorists also by land at the most convenient time for us,” Turkish President Recep Tayyip Erdogan said late on Wednesday.

“Turkey has the power to identify, catch and punish terrorists who are involved in attacks against our country and nation, and those helping them, inside and outside our borders."

Turkey will, step by step, establish a so-called “safe zone” in the north-east, Mr Erdogan said.

If the ground offensive goes ahead it will be the fourth major military incursion Turkey has carried out in Syria since 2016.

Mr Erdogan has also launched air strikes against Kurdish militia groups in Iraq.

Who does Turkey consider to be the biggest Kurdish militia in Syria?

The latest attacks are seen as part of a long-running Turkish campaign in Iraq and Syria against the Kurdistan Workers Party, known as the PKK, and the Syrian Kurdish People’s Protection Units (YPG).

Turkey considers the YPG a terrorist group, insisting it is an extension of the PKK which has fought for Kurdish autonomy in Turkey for decades.

The PKK has also been fighting a decades-long insurgency against Turkey, while Kurdish militant groups control some areas of northern Syria close to the Turkish border.

Another group known as the Syrian Democratic Forces (SDF) is an alliance of Kurdish and Arab militias founded in 2015, nearly four years after an armed rebellion erupted against Syrian President Bashar Al Assad.

The SDF became a key US ally in the fight against ISIS and is an alliance dominated by the YPG.

The alliance has denied involvement in this month's Istanbul attack and accused Turkey of using the bombing as a pretext to launch a long-planned cross-border offensive.

Ankara says Kurdish militant groups are a threat to its national security and plans to resettle in Turkey any Syrian refugees from areas it conquers.

What is the PKK?

The PKK was formed in 1978 by Abdullah Ocalan, a radical Marxist who found support from the Syrian government under former president Hafez Al Assad.

The group took up arms in 1984, waging an insurgency against Turkey from its bases in the south-eastern part of the country as well as from northern Iraq.

For the PKK, the purpose of the conflict against Ankara has been to gain greater cultural and political rights for the Kurdish people, initially with the objective of establishing an independent state. In later years, however, demands have shifted to focus on greater Kurdish autonomy within Turkey.

There are approximately 30 million Kurds living in the Middle East, primarily in Iran, Iraq, Syria and Turkey. They make up nearly one fifth of Turkey’s population of 79 million.

The PKK has long used the rugged terrain of northern Iraq as a rear base from which to stage attacks against Turkey.