Turkish forces have launched a military offensive against Kurdish YPG fighters in Syria's northwestern region of Afrin, opening a new front in the nearly seven-year-old Syrian war.
The campaign, using air power, artillery and ground troops, has alarmed the United States which has supported the YPG, or People's Protection Units, in fighting Islamic State in northern Syria.
Why is Turkey attacking Afrin?
Syrian government forces withdrew from Afrin shortly after the start of Syria's civil war and - like a much larger swathe of Kurdish controlled land further east - the Kurdish region has been autonomous for several years.
Turkey, which has battled a decades-old insurgency in its mainly Kurdish southeast, considers the Syrian Kurdish YPG which controls Afrin to be a terrorist group whose growing power on its southern border threatens Turkish security.
Ankara was angered by the United States decision to ally itself with the YPG against Islamic State, and President Tayyip Erdogan has repeatedly said Turkey would act to crush the "terror corridor" of Kurdish territory stretching along much of the 900 km (560 mile) border.
The YPG has said it wants good neighbourly relations with Turkey.
Erdogan launched the Afrin offensive less than a week after the announcement of U.S. plans to train a 30,000-strong border force in the area of northern Syria where YPG-dominated forces drove out Islamic State fighters last year.
That infuriated Erdogan, who promised to "strangle" the force before it came into existence.
U.S. officials later said the announcement had been misleading and there were no plans for a formal border force, but confirmed that Washington did want to support local fighters to prevent a resurgence of Islamic State.
Why does the U.S. care?
While the United States says it has not supported the YPG fighters in Afrin, a relatively small pocket of territory in the northwest of Syria, it has 2,000 military personnel in the separate and much larger area of northern Syria where the YPG is present.
Erdogan has said Turkey will also target the mainly Arab town of Manbij, part of the larger area controlled by the YPG and its allies.
That would pit soldiers from Turkey against fighters armed and supported by its NATO partner the United States - and possibly against U.S. troops, some of whom are in the Manbij area, according to U.S. officials.
Conflict with the YPG, especially if it extends to Manbij, would also threaten U.S. hopes to stabilise the area of Syria recaptured from Islamic State by the YPG-led forces.
What about Russia?
Last week Turkey sent its chief of military staff to Russia, which controls much of western Syria's air space and is President Bashar al-Assad's most powerful international ally, to discuss using the air space for its Afrin operation.
Erdogan later said Ankara and Moscow had an agreement on the military operation.
Moscow has not confirmed any deal, but the YPG said Russia had "opened the skies of Afrin" to Turkish warplanes. Russia also withdrew a military team from the region, Russian media said.
While Russia has joined other major powers in calling for restraint, Foreign Minister Sergei Lavrov appeared to blame Washington, saying U.S. "unilateral actions" in Syria had infuriated Turkey.
Ahead of the Turkish assault, the Syrian government said it was ready to target Turkish jets in its air space.
But Damascus has not followed through on that threat. It has condemned Turkey's incursion, but not sent troops to Afrin to support Kurdish fighters it suspects of wanting independence from Damascus in the long run.
What does it mean for the Syrian conflict?
If Turkey and its Free Syrian Army rebel allies defeat the YPG in Afrin they would link up two areas of rebel-held territory in northwest Syria - the Idlib region west of Afrin and another area to the east which Turkish-backed rebels control after Ankara's "Euphrates Shield" military incursion in 2016.
That would be a boost for Assad's opponents, who have been losing ground for the last two years to government forces supported by Russian air power and Iranian-backed militias.
But it would also confront the anti-Assad Syrian rebels with the predicament of governing a large Kurdish population.
Before launching the operation, Turkey's foreign minister said Ankara had discussions with Russia and Iran, highlighting the extent of Moscow and Tehran's influence in a conflict which started with local protests against Assad and grew into a proxy war for foreign powers.
How long could the operation last?
Six days into the campaign, Turkish soldiers and their FSA rebel allies have been fighting to gain footholds on the western, northern and eastern flanks of Afrin.
They appear to have made only limited progress, since rain and cloud have hampered air support.
Erdogan's government has sent mixed signals about the duration of the campaign. The deputy prime minister responsible for economic affairs sought to reassure investors by saying it would be over quickly.
Other ministers have suggested a more open-ended campaign, pushing back against international calls for restraint and for Turkey to put a time limit on the conflict.
Erdogan's spokesman said operations in Syria would continue until the 3.5 million Syrian refugees in Turkey could return.
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