US Senate majority leader Mitch McConnell has attacked US President Donald Trump's decision to pull troops from Syria as "a strategic nightmare" as scattered fighting flared in the north of the country despite a ceasefire deal.

Turkey had agreed to suspend its Syria offensive for five days but President Recep Tayyip Erdogan warned on Friday he would resume a full-scale operation against Kurdish forces if they do not withdraw from a border "safe zone".

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Trump said Erdogan told him there had been "minor sniper and mortar fire" in the region "that was quickly eliminated" and the Turkish leader assured him in a call that "he very much wants the ceasefire, or pause, to work".

Mustefa Bali, a spokesperson for the Kurdish Syrian Democratic Forces (SDF), accused Turkey, however, of violating the ceasefire deal reached during a visit to Ankara on Thursday by US Vice President Mike Pence.

"Despite the agreement to halt the fighting, air and artillery attacks continue to target the positions of fighters, civilian settlements and the hospital" in the border town of Ras al-Ain in northe-astern Syria, he said.

'Grave strategic mistake'

McConnell on Friday said Trump's decision to withdraw US troops would help Washington's foes and hurt its allies.

"Withdrawing US forces from Syria is a grave strategic mistake," McConnell, the top Republican in Congress, wrote in The Washington Post.

"It will leave the American people and homeland less safe, embolden our enemies, and weaken important alliances."

The deal brokered by Pence was meant to provide a pause for the evacuation of Kurdish fighters from a "safe zone" Turkey wants to control along its border with Syria. Ankara considers the Kurdish forces to be "terrorists" linked to Kurdish rebels inside Turkey.

"If the promises are kept until Tuesday evening, the safe zone issue will be resolved," Erdogan told reporters in Istanbul.

"If it fails, the operation... will start the minute 120 hours are over."

The suspension of hostilities looked designed to help Turkey achieve its main territorial goals without fighting, but its Syrian proxies continued to clash with Kurdish fighters on Friday.

Fourteen civilians were killed in Turkish air strikes and mortar fire by allied Syrian fighters in and around the village of Bab al-Kheir, the Syrian Observatory for Human Rights said.

The Britain-based war monitor said eight fighters of the SDF - the de facto army of the embattled Kurdish autonomous region - also died.

US Secretary of State Mike Pompeo said on Friday there was also "some very positive activity".

"And we're hopeful in the hours ahead that both the Turks who were part of the agreement alongside of us as well as the YPG fighters in the region will take seriously the commitments they made," he told reporters in Brussels.

Under the deal, Kurdish forces are required to withdraw from a border strip 32km deep.