SOCHI, Russia/ANKARA — Hundreds of Kurdish fighters remain near to Syria's northeast border despite a US-brokered truce demanding their withdrawal and Turkey could resume its offensive in the area when the cease-fire expires, Turkish President Tayyip Erdogan said.

The five-day truce in Turkey's cross-border offensive to allow the withdrawal of Kurdish YPG fighters from the border area ends at 10 p.m. (1900 GMT) on Tuesday.

Turkey says Kurdish YPG militia forces, which it views as terrorists because of their links to Kurdish militants waging an insurgency in southeast Turkey, must leave a "safe zone" it wants to establish inside Syria.

"The withdrawal is continuing," Erdogan told reporters at Ankara airport before flying to Russia for talks on Syria with President Vladimir Putin, who supports the Damascus government and has also sent troops to northeast Syria.

"We are talking about 700-800 (YPG fighters) already withdrawn and the rest, around 1,200-1,300, are continuing to withdraw.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 criticized by US lawmakers, including some of Trump's fellow Republicans, as a betrayal of Kurdish allies who have helped the United States fight Daesh (the so-called IS) 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.

Turkey says it wants to set up a "safe zone" along 440 km (275 miles) of border with northeast Syria, but its assault so far has focused on two border towns in the center of that strip, Ras Al-Ain and Tel Abyad, about 120 km apart.

A Turkish security source said initially the YPG was pulling back from that 120 km 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 Turkey's 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.

The Kremlin said it hoped Erdogan would be able to provide Putin with more information about Ankara's plans for northeast Syria, and was also studying what it described as a new idea from Germany for an internationally controlled security zone in northern Syria involving Turkey and Russia.

German Defense Minister Annegret Kramp-Karrenbauer said the step should stabilize the region so that civilians could rebuild and refugees could return on a voluntary basis.

Russia is a close ally of Syrian President Bashar 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.

Ankara is holding covert contacts with Damascus, partly via Russia, to avert direct conflict in northeast Syria, Turkish officials say, although publicly hostility between the two governments remains.

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.

The US withdrawal has left a vacuum into which Turkish forces have pressed in from the north, while from the southwest Russian-backed Syrian troops have swept back into territory they were driven from years ago.

On Monday, US Defense Secretary Mark Esper said the Pentagon was considering keeping some US troops near oilfields in northeastern Syria alongside Kurdish-led Syrian Democratic Forces (SDF) to help deny oil to Daesh militants.

There was confusion on Tuesday over the status of the nearly 1,000 troops pulling out of Syria into neighboring Iraq.

The Pentagon had said they were expected to move to western Iraq to continue the campaign against Daesh, but the Iraqi military said they had no permission to stay in Iraq. Esper later said they would not stay in Iraq "interminably". — Reuters