Kurdish forces are continuing to withdraw in northeast Syria but Turkey will resume its military assault there once a United States-brokered ceasefire expires on Tuesday if promises given by Washington are not kept, President Tayyip Erdogan said.

A five-day pause in Turkey's cross-border military offensive to allow the withdrawal of Kurdish YPG fighters from the border area expires at 10pm (local time) on Tuesday.

Turkey says Kurdish YPG militia forces must leave a "safe zone" it wants to establish along its border with northeast Syria. Ankara views the YPG as terrorists with links to Kurdish insurgents operating in southeast Turkey.

"The withdrawal is continuing," Erdogan told reporters at Ankara airport before flying to Russia for talks with President Vladimir Putin on Syria.

"According to the information I have received from my defence minister we are talking about 700-800 already withdrawn and the rest, around 1,200-1,300, are continuing to withdraw. It has been said that they will withdraw," Erdogan said.

"All will have to get out, the process will not end before they are out."

Turkey began its cross-border operation nearly two weeks ago following US President Donald Trump's decision to withdraw American troops from northern Syria.

The American withdrawal from Syria has been criticised by US lawmakers, including some of Trump's fellow Republicans, as a betrayal of Kurdish allies who have helped the US fight the militant Islamic State group in Syria.

Trump said on Monday it appeared that the five-day pause was holding despite skirmishes, and that it could possibly go beyond Tuesday's expiry, but Erdogan said the fighting may resume.

"If the promises given to us by America are not kept, we will continue our operation from where it left off, this time with a much bigger determination," he said.

'Safe zone'

Turkey says it wants to set up a "safe zone" along 440 kilometres of border with northeast Syria, but its assault so far has focused on two border towns in the centre of that strip, Ras al Ain and Tel Abyad, about 120km apart.

A Turkish security source said the YPG was initially pulling back from the 120km border strip. He said Erdogan and Putin would discuss a wider withdrawal from the rest of the border in their talks on Tuesday in the Russian Black Sea resort of Sochi.

Syrian and Russian forces have already entered two border cities, Manbij and Kobani, which lie within Turkey's planned "safe zone" but to the west of the current military operations.

Erdogan has said he could accept the presence of Syrian troops in those areas, as long as the YPG are pushed out.

"My hope is that God willing we will achieve the agreement we desire," he said before leaving for Sochi.

Russia is a close ally of Syrian President Bashar al-Assad. Turkey has backed rebels seeking to oust Assad during Syria's more than eight-year-long civil war but has dropped its once-frequent calls for Assad to quit.

Turkey is holding covert contacts with Syria's government, partly via Russia, to avert direct conflict in northeast Syria, Turkish officials say.

Some 300,000 people have been displaced by Turkey's offensive and 120 civilians have been killed, according to the Syrian Observatory for Human Rights, a UK-based war monitor. It said on Sunday 259 fighters with the Kurdish-led forces had been killed, and 196 Turkey-backed Syrian rebels.

Turkey says 765 terrorists but no civilians have been killed in its offensive.