Turkey v Syria's Kurds v Islamic State
Turkey has been shelling both Kurdish fighters of the Popular Protection Units (YPG) and so-called Islamic State inside northern Syria.
The YPG are widely regarded as one of the most effective forces in the fight against IS and attacks on them have prompted calls from the UN, US and EU for Turkey to show restraint.
Ankara says it is retaliating against what it calls provocations by the YPG but it has long warned against the group making territorial advances in northern Syria near its border.
Why is Turkey hostile to the YPG?
Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), a Turkish-Kurdish rebel group fighting for autonomy since the 1980s.
A ceasefire between Turkey and the PKK ended last year and clashes have since claimed hundreds of lives on both sides.
While Turkey is fighting against the PKK on its soil, neighbouring Syrian Kurds continue carving out a self-declared autonomous region known as Rojava, and the YPG has gained the trust of Turkey 's international allies in the fight against IS.
What does the international community say?
The PKK is officially considered a terrorist organisation by the EU and the US but Turkey has failed to convince its Western allies to regard the YPG likewise.
The US state department and Pentagon have in the past pressed Turkey to stop shelling the YPG and urged Syrian Kurds not to expand their areas of operation.
The EU has also called on Turkey to stop shelling Syrian territory and refrain from further complicating the efforts of world powers to halt the hostilities.
The UN has urged Turkey to comply with international law in Syria.
What do Syria's Kurds say?
A US-backed alliance between the YPG and some Syrian Arabs called the Syrian Democratic Forces (SDF) has been advancing and taking over strategic areas from various armed Islamist groups despite Turkey 's ongoing bombardments.
It scored a notable success by driving IS out of the crossroads town of Manbij this month.
The jihadists were forced to retreat to the town of Jarablus on the Turkish border, which is believed to be the next focus of the advance against IS.
Turkey is wary of the Kurds trying to take control of another border town to the west, Azaz, which it sees as part of a planned "buffer zone" in northern Syria.
Why do Kurds accuse Turkey of backing IS?
Kurds accuse Turkey of using the US-led coalition against IS as a cover to attack the PKK in both Turkey and Iraq, and now against the YPG in northern Syria.
The Kurds say Turkey's bombardment of their positions is helping IS to attack Kurdish-held frontline areas in Syria and Iraq.
Turkish bombardments have also angered Iraqi Kurds, since IS has increased its attacks against Iraqi Kurdish forces in the Makhmur area near the city of Mosul.
"Turkey shamelessly and openly backs IS and al-Qaeda terrorists against Kurdish freedom fighters," Sadi Pria, a top Iraqi Kurdish official in Irbil, has said.
What role does the PKK have in all this?
Syrian Kurds, including those leading the YPG, share the PKK's aspiration to establish an autonomous democratic confederation but the YPG denies Turkey's claim that it is an extension of the PKK.
PKK leaders have repeatedly warned of a "total civil war" inside Turkey in retaliation for attacks against Syria 's Kurds.
Meanwhile, the Turkey-PKK battlefield has expanded from guerrilla fighting in the mountains to urban warfare in the heart of Turkey 's predominantly Kurdish cities, including in Diyarbakir .
Where is this heading?
Turkey says it will continue to bombard the Syrian Kurds as long as it feels threatened by them.
The YPG will most probably ignore Turkey 's threats and continue its advances to capture more areas. In July 2015, the group took over the strategic border town of Tal Abyad despite Turkey 's warnings not to do so.
On the other side of the border an escalation of the Turkey-PKK conflict is highly likely in the form of urban and rural warfare, especially after winter.