The truce was announced by US Vice President Mike Pence after talks in Ankara with Turkey's President Tayyip Erdogan, and was praised by President Donald Trump, who said it would save "millions of lives".
But if implemented it would achieve all the main objectives Turkey announced when it launched its assault on October 9: control of a strip of Syria more than 30 km (20 miles) deep, with the Kurdish militia, once US allies, obliged to pull out.
Trump tweeted: "Great news out of Turkey".
It was also unclear if the Kurdish-led Syrian Democratic Forces (SDF) would fully comply with the agreement, which would leave Turkish forces in charge of a swathe of territory that the Kurds once held with US military support.
SDF commander Mazloum Kobani told Ronahi TV the group would accept the ceasefire agreement with Turkey in northern Syria but said it was limited to the border areas running between the towns of Ras al-Ain and Tal Abyad.
What do US senators say?
Republican and Democratic senators accused Trump of having betrayed the Kurdish allies who were vital in fighting Islamic State militants, of brushing aside the humanitarian costs of Turkey's invasion and of being outwitted by Ankara.
"The safe zone will be primarily enforced by the Turkish Armed Forces," a joint US-Turkish statement released after the talks said.
US 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 despite the ceasefire announcement.
A pause, not ceasefire!
A Turkish official told Reuters that Ankara got "exactly what we wanted" from the talks with the United States. Turkish Foreign Minister Mevlut Cavusoglu described it as a pause, solely to allow the Kurdish fighters to withdraw.
Kurdish fighters would be forced to give up their heavy weapons and their positions would be destroyed, Cavusoglu said. He declined to call the agreement a "ceasefire", saying ceasefires could be agreed only by legitimate sides, and not by a Kurdish militia that Turkey considers a terrorist group.
"When the terrorist elements completely leave the safe zone, we can stop the operation," Cavusoglu said.
Tackling Islamic State
Washington and Ankara will cooperate to handle Islamic State (IS) fighters and their families held in prisons and camps, the joint statement said, addressing concerns that the militant group might reconstitute and again attack Western targets.
Kurdish response remains uncertain
Pence said Washington had already been in contact with the Kurdish-led SDF, which had agreed to withdraw and were already pulling out.
However, the Kurdish position was not clear. Speaking to Ronahi TV, SDF commander Kobani said the agreement is "just the beginning" and will not achieve Turkey's goals.
Aldar Xelil, a leading Syrian Kurdish politician, told Al Arabiya television that the Kurds would abide by the ceasefire but would defend themselves.
Pence said that once the pause became permanent, Washington would go ahead with its own plans to withdraw its forces from northern Syria, which had partnered with the Kurds to fight against Islamic State. However, there were no signs that US forces had ceased their withdrawal, a US official said.
No Turkish engagement in Syrian border flashpoint
The deal struck with Erdogan also provided for Turkey not to engage in military operations in the flashpoint Syrian border town of Kobani, Pence said. Cavusoglu said Turkey had given no commitments about Kobani.
US Special Representative for Syria Engagement James Jeffrey said the agreement covered central northeastern Syria and Turkey was in separate talks with the Russians and the Syrians about other parts of the region.
What happens to the Kurds?
Senator Mitt Romney, a Republican, said the agreement "is far from a victory" and demanded the administration explain what will happen to the Kurds, what will be the future US role in the region and why Turkey "will face no apparent consequences."
Turkey begets fresh crisis in Syria
The Turkish assault has created a new humanitarian crisis in Syria with 200,000 civilians taking flight, a security alert over thousands of Islamic State fighters potentially abandoned in Kurdish jails, and a political maelstrom at home for Trump.
The Turkish assault began on October 9 after Trump moved US troops out of the way after an October 6 phone call with Erdogan. Trump announced sanctions on Turkey on Monday, after the assault began, but critics said these were too little, too late.
Pence said the sanctions would be lifted once the ceasefire became permanent.