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.