Hundreds of civilians are reported to have been displaced by fierce clashes between Turkish-led forces and Kurdish fighters on Syria’s northern border.
The UK-based Syrian Observatory for Human Rights said people were fleeing the Kurdish enclave of Afrin and heading towards government-held Aleppo.
But it cited sources as saying they were being prevented from crossing into the city’s mainly Kurdish districts.
The Turkish military is trying to drive a Kurdish militia out of Afrin.
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Ankara says the People’s Protection Units (YPG) is an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in south-eastern Turkey for three decades. Turkey considers the YPG a terrorist group.
The YPG denies any direct organisational links to the PKK – an assertion backed by the US, which has provided the militia and allied Arab fighters with weapons and air support to help them battle the jihadist group Islamic State (IS) in Syria.
What is happening on the ground?
Turkish troops and allied Syrian rebels so far appear to have made limited gains since the air and ground offensive began on Saturday.
On Tuesday, the Turkish state-run Anadolu news agency reported that rebels had captured the “strategic” village of Hammam in north-western Afrin, as well as two hills. It also reported that a second Turkish soldier had been killed in clashes.
YPG sources told the Kurdish Hawar news agency that its fighters had foiled an attempt to seize the village of Qastal Jando, killing many attackers.
Kurdish leaders also invited civilians to take up arms, saying the YPG was “ready to receive all those who wish to defend Afrin and provide them with weapons”.
The Syrian Observatory reported that thousands of civilians were trying to flee to Aleppo, about 50km (30 miles) from the town of Afrin. However, it cited unnamed sources as saying people were being stopped at government checkpoints and prevented from reaching the homes of relatives and friends in two Kurdish districts.
There was no immediate comment from the government, which has mostly avoided conflict with the Kurds.
The UN said it was deeply concerned by the fighting, and warned that an estimated 450,000 civilians in Afrin were at risk. About 126,000 people living in the enclave had already been displaced and were at risk of further upheaval, it added.
“We have been preparing for a large scale response depending on the needs of the civilian population,” spokesman Stéphane Dujarric told reporters. “We cannot stress enough the need for all parties involved to protect civilians, to protect civilian infrastructure, and to respect international law.”
How many people have died?
Neither side has released much information about fatalities, making the death toll difficult to gauge.
The Syrian Observatory reports that 25 Syrian rebels and 26 Kurdish fighters have been killed so far.
It also says 24 civilians have been killed – 22 in Turkish air and artillery strikes, and two as a result of Kurdish fire.
Turkey has denied killing civilians, saying it only targets combatants.
What is the background to the offensive?
Turkish President Recep Tayyip Erdogan has vowed to crush the YPG, which controls Afrin and more than 400km (250 miles) of Syria’s northern border.
He has vehemently opposed Western support for the militia and launched the assault on Afrin – dubbed “Operation Olive Branch” – not long after news reports said the US had begun helping the YPG form a “border security force”.
Turkey’s prime minister said it wanted a 30km-deep (19-mile) “safe zone” along its border inside Syria, which would encompass most of the Afrin enclave.
About 10,000 Syrian rebels are reportedly taking part in the offensive, while Turkish officials estimate that there are between 8,000 and 10,000 YPG fighters in Afrin.
How have key powers reacted?
On Tuesday, US Defence Secretary Jim Mattis said he took “very seriously Turkey’s legitimate security concerns”, but urged it to “exercise restraint in the military action and the rhetoric”.
“The violence in Afrin disrupts what was a relatively stable area in Syria and distracts from the international effort to defeat [IS],” he added.
Russia – an ally of the Syrian government that has withdrawn military observers from Afrin and effectively allowed Turkish warplanes to fly over the enclave – called for restraint and respect for Syria’s sovereignty and territorial integrity.
Politicians in Germany, meanwhile, reacted angrily to Turkey’s use of German-made Leopard tanks. The chairman of the parliamentary foreign affairs committee said Germany should not carry out a planned upgrade of the tanks for its Nato ally.