Water to Syria's Afrin has now been cut for a week, the United Nations said on Wednesday, as Turkey's military finished encircling the Kurdish city in preparation for a major ground assault.

Access to clean drinking water ended after Turkish troops and allied Syrian fighters seized the main dam and water plant from the People's Protection Units (YPG) militia in the Kurd-dominated region in northwest Syria.

"Local workers were unable to access the dam controls to pump water. The water supply has been cut off," the UN office for humanitarian affairs said in a statement.

Residents have relied on untreated water from boreholes and risk contracting diseases, it added.

The Syrian-Kurdish YPG said the Turkish army took control of the dam on Maydanki lake and a water pumping station last week.

"Water has been fully cut off because of the Turkish army's control over it," said Birusk Hasaka, the YPG spokesman in Afrin.

"Civilians are depending on wells to get water and they are not enough - and not good for drinking, unfortunately."

Without bread

The area also faces a bread shortage as Afrin's only bakery grapples with rising demand amid power and fuel shortages.

Azad Mohamed, an Afrin resident, said he waited in line for eight hours to get a few loaves of bread. Some people went without getting any, he added.

Turkey's President Recep Tayyip Erdogan said his military and its rebel allies would surround Afrin city by Wednesday night, a source in his office said.

"The circle of encirclement will be completed by tonight," said the presidential source.

He said planning for the capture of Afrin was being done strategically to prevent civilian deaths.

"We will clear Afrin of terrorists, clear Manbij, and east of the Euphrates River to our border with northern Iraq," state-run Anadolu news agency quoted Erdogan as saying.

The United States has about 2,000 soldiers based in Manbij - about 100km east of Afrin - who work with the YPG.

Erdogan has urged the US to pull out of the area, but American military officers say they have no intention of doing so.

'Care taken'

The UK-based war monitor, the Syrian Observatory for Human Rights has estimated there are about one million civilians in and around Afrin city.

"All care is being taken. Right now the first civilians are being taken out of Afrin in vehicles through a special corridor," Erdogan was quoted as saying by the Hurriyet daily.

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He added that if humanitarian concerns had been ignored, the operation against the YPG would "already be over."

According to Erdogan, the cross-border offensive has captured nearly 1,300-square kilometres of territory from the US-backed Kurdish fighters.

About 3,500 "terrorists have been neutralised" during the 54-day operation that was launched on January 20, he said.

Since the start of Syria’s conflict in 2011, the YPG and its allies have set up three autonomous regions in the north, including Afrin.

Washington's assistance for the Kurdish force infuriated Turkey. Ankara sees the YPG as an extension of the outlawed Kurdistan Workers' Party (PKK), which has waged a decades-long armed rebellion in Turkey that has killed an estimated 40,000 people.