Turkish forces and allied rebels surrounded the Syrian city of Afrin on Tuesday, leaving hundreds of thousands of its mostly Kurdish residents trapped.
Hundreds of civilians managed to flee advancing troops on Monday, but an estimated 700,000 are now encircled in Afrin and nearby villages and towns.
The only road out was in range of Turkish artillery fire and was therefore impassable, the UK-based Syrian Observatory for Human Rights said.
Some 230 civilians have been killed so far in Turkey's Operation Olive Branch, however Kurdish fighters warned the new siege could trigger a humanitarian crisis.
There was no water after Turkish forces seized control of the local dam, and residents say they have relied on water wells for their consumption.
Syria - Operation Olive Branch
“Where is the international community? Why don’t they cry tears for all Syrian civilians, not only some?” Ahmed Murad, a resident of Afrin told the Telegraph. “They are too scared to criticise their Turkish ally.”
While attention in recent weeks has focused on a ferocious regime assault on rebel-held Eastern Ghouta, in Syria's north Turkish forces and Syrian opposition fighters have been advancing in their offensive against the Kurdish enclave.
Turkey launched its operation on January 20, saying it intended to clear the border of YPG fighters, whom they consider terrorists. However, its mission has expanded to include territory deep into Syria.
Senior officials have said that Turkey was trying, through war, to take lands it occupied during the Ottoman Empire.
Redur Xelil, head of foreign relations for the Syrian Democratic Forces, a US-backed alliance of fighters that includes the YPG, accused the Turkish forces of carrying out "demographic change" in Kurdish territory captured in Afrin.
"The Turkish government is settling Turkmen and Arab families in the villages of Afrin that it occupied after forcing out its people, and is distributing the belongings of the people of Afrin to the new settlers," he said.
The United Nations said that it received "disturbing reports" of civilian deaths in the northwestern Syrian enclave and that it believes "tens of thousands" have been displaced.
Turkey denied the claims.
The Turkish offensive opened a new front in a multi-sided civil war now entering its eighth year, which has drawn in neighbours and global powers who have armed and assisted fighters on the ground.
Turkey considers the YPG a serious threat to its security, while the US has relied on the group in the fight against the Islamic State of Iraq and the Levant (Isil).
Their different positions have been the source of increasing friction between the Nato allies.
Ankara is riled by US support and arming of the Kurds, and accuses Washington of not fulfilling a promise to move the Syrian Kurdish fighters in Manbij east of the Euphrates River.
It threatened on Tuesday to expand its current offensive in northern Syria to drive the Syrian Kurds out of Manbij, where the US has troops.
Mevlut Cavusoglu, Turkey’s foreign minister, said it had been agreed that the YPG would leave Manbij and Turkey and the US would jointly secure the area.
The US did not immediately confirm and the YPG said it had not been informed of any such deal.