Reuters journalists at the border heard machine-gun fire and shelling and saw smoke rising from the Syrian border battlefield city of Ras al Ain, although the sounds of fighting had subsided by mid-morning.
The truce, announced on Thursday by U.S. Vice President Mike Pence after talks in Ankara with Turkish President Tayyip Erdogan, sets out a five-day pause to let the Kurdish-led SDF militia withdraw from an area controlled by Turkish forces.
The SDF said air and artillery attacks continued to target its positions and civilian targets in Ral al Ain.
"Turkey is violating the ceasefire agreement by continuing to attack the town since last night," SDF spokesman Mustafa Bali tweeted.
The Kurdish-led administration in the area said Turkish truce violations in Ras al Ain had caused casualties, without giving details.
The deal was aimed at easing a crisis that saw President Donald Trump order a hasty and unexpected U.S. retreat, which his critics say amounted to abandoning loyal Kurdish allies that fought for years alongside U.S. troops against Islamic State.
Trump has praised the deal, saying it would save "millions of lives." White House spokeswoman Stephanie Grisham told Fox News the ceasefire was successful even if halting fighting "takes time."
Turkey cast it as a complete victory in its campaign to control a strip of territory stretching hundreds of miles along the border and more than 30 km (around 20 miles) deep into Syria, to drive out fighters from the YPG, the SDF's main Kurdish component.
"As of now, the 120-hour period is on. In this 120-hour period, the terrorist organization, the YPG, will leave the area we identified as a safe zone," Erdogan told reporters after Friday prayers in Istanbul. The safe zone would be 32 km deep, and run "440 km from the very west to the east," he said.
But the U.S. special envoy for Syria, James Jeffrey, said the agreement covered only a smaller area where Turkish forces were already operating, without giving details of how far along the border Washington believed it stretched.
The Kurds said it was limited to a small strip between two border towns that have seen the bulk of the fighting: Ras al Ain and Tal Abyad, just 120 km away.
RUSSIA, IRAN FILL VACUUM
With the United States pulling its entire 1,000-strong contingent from northern Syria, the extent of Turkey's ambitions is likely to be determined by Russia and Iran, filling the vacuum created by the U.S. retreat.
The government of President Bashar al-Assad, backed by Moscow and Tehran, has already taken up positions in territory formerly protected by Washington, invited by the Kurds.
Jeffrey acknowledged that Turkey was now negotiating with Moscow and Damascus over control of areas where Washington was pulling out, which were not covered by the U.S.-Turkish ceasefire agreement.
"As you know we have a very convoluted situation now with Russian, Syrian army, Turkish, American, SDF and some Daesh (Islamic State) elements all floating around in a very wild way," Jeffrey said.
"Now, the Turks have their own discussions going on with the Russians and the Syrians in other areas of the northeast and in Manbij to the west of the Euphrates," he said. "Whether they incorporate that later into a Turkish-controlled safe zone, it was not discussed in any detail."
LIFTING SANCTIONS?
The joint U.S.-Turkish statement released after Thursday's talks said Washington and Ankara would cooperate on handling Islamic State fighters and family members held in prisons and camps, an important international concern.
Pence said U.S. sanctions imposed on Tuesday would be lifted once the ceasefire became permanent.
In Washington, U.S. senators who have criticized the Trump administration for failing to prevent the Turkish assault in the first place said they would press ahead with legislation to impose sanctions against Turkey.
The Turkish assault began after Trump moved U.S. troops out of the way following an Oct. 6 phone call with Erdogan.
It has created a new humanitarian crisis in Syria with - according to Red Cross estimates - 200,000 civilians taking flight, a security alert over thousands of Islamic State fighters potentially abandoned in Kurdish jails, and a political storm at home for Trump.
Turkey says the "safe zone" would make room to settle up to 2 million Syrian refugees it is currently hosting, and would push back the YPG militia which it deems a terrorist group because of its links to Kurdish insurgents in southeast Turkey.
A Turkish official told Reuters that Ankara got "exactly what we wanted" from the talks with the United States.