ISTANBUL - President Recep Tayyip Erdogan warned Friday that Turkey's military offensive would resume within days if Kurdish fighters do not fully withdraw from a buffer zone even as ongoing violence in at least one Syrian border town imperiled a cease-fire brokered by the United States.

There were also indications on Friday that the United States and Turkey had not come away from their negotiations with the same understanding of what they had agreed to after signing the vaguely written deal.

"If the U.S. can keep its promise made to us, at the end of the 120 hours, the issue of the safe zone will be resolved," Erdogan told a gathering of foreign news media in Istanbul, referring to the length of a pause in the offensive agreed upon by Turkey and the United States on Thursday.

"If the promises are not kept," he added, "our operation is going to continue from where we left off."

Turkey agreed to pause its eight-day military operation after Vice President Mike Pence led a U.S. delegation to Ankara on Thursday and met with Erdogan. A 13-point agreement said Turkey would permanently halt the offensive after 120 hours - or five days - if Syrian Kurdish militias, known as the Syrian Democratic Forces (SDF), withdraw from an area of northern Syria that Turkey refers to as a "safe zone."

The agreement amounted to a stunning victory for Erdogan, who not only convinced the Trump administration to accept the "safe zone" - a concept that Washington and Ankara have argued about for years - but also made the United States the guarantor of the Kurdish fighters' withdrawal.

Turkey views the Syrian Kurdish militias as terrorists because of their ties to the insurgent Kurdistan Workers' Party (PKK) in Turkey. The "safe zone" proposal is aimed at pushing the militias farther away from Turkey and creating an area where Turkey proposes to resettle hundreds of thousands of Syrian refugees.

The challenges of enforcing the agreement quickly became apparent Friday as the SDF accused Turkey of failing to observe the cease-fire. Mervan Qamishlo, a spokesman for the SDF, said that it was not withdrawing yet from the buffer zone "because the Turkish side has not yet committed to the agreement" and was still bombing the border town, Ras al-Ayn, including the town's hospital.

Smoke could be seen rising from Ras al-Ayn in footage broadcast by CNN early Friday. Journalists for the Associated Press reported shelling and the sound of gunfire.

Erdogan denied Friday that violence was continuing in the area. President Donald Trump, in a tweet Friday afternoon, said Erdogan told him in a phone call that "there was minor sniper and mortar fire that was quickly eliminated." Trump added: "He very much wants the ceasefire, or pause, to work."

The National Army, a Turkish-backed umbrella group that united Syrian rebel factions ahead of the offensive, also denied that there was any fighting or shelling in Ras al-Ayn.

A civilian near Ras al-Ayn said the bombardment continued through the night and into Friday morning. "There have been no airstrikes so far, but neither the bombardment nor the clashes have ceased," he said in a telephone interview, declining to give his name for security reasons. "We are among the few dozens of families that have stayed in the area, but we have our car ready and we may still leave, despite the cease-fire."

He said most residents had already fled, adding, "I don't think they are going to come back."

As part of the agreement, the White House agreed to refrain from imposing new economic sanctions on Turkey, and to reverse sanctions that were imposed earlier this week, once "a permanent cease-fire was in effect," Pence said. But other unresolved issued remained.

It was not clear whether Turkey and the United States had come to an understanding about the size of the safe zone, which is not laid out in the text of the agreement. U.S. and Turkish officials concur that it will extend about 20 miles south of the Turkish border. But they say different things about its width.

Erdogan said Friday the area stretched for 275 miles, from Jarabulus to the Iraqi border, pointing at a map displayed behind him that showed a corridor stretching across much of northern Syria. Asked if the Trump administration had agreed, he said: "This is what we proposed. They did not say anything negative vis a vis the proposal."

A senior U.S. official said that Erdogan had "always defined the 'safe zone' in that way. We never have." According to the official, the Kurdish fighters would retreat from a zone in a much smaller area between the Syrian border towns of Tel Abyad and Ras al-Ayn, which are roughly 60 miles apart in the middle of northeastern Syria. "As the Turks have agreed," said the official, who was granted anonymity to discuss sensitive negotiations.

The safe zone will be "primarily enforced by the Turkish Armed Forces," the agreement states. A U.S. military spokesman said Friday that U.S. forces would not patrol the safe zone.

Erdogan said Friday that Turkey did not intend to keep its forces in Syria "forever."

"Not at all," he said. "What we want is to make sure this area is cleared of terrorist elements, and at least 1 million refugees in Turkey can return to their own villages." At the same time, the map he displayed for journalists showed a line of blue boxes in the "safe zone" corridor - sites where Turkey would set up "observation posts," Erdogan said.

The Turkish offensive rattled Ankara's western allies and put Trump on the defensive, after he was criticized for the perception he approved Erdogan's military operation during a phone call between the two leaders a few days before the offensive began. The White House denied there had been any green light and released a toughly worded letter Trump sent to Erdogan, on the day of the offensive, showing that Trump tried to persuade the Turkish leader to relent.

"Don't be a fool!" the letter concluded.

Erdogan repeatedly praised Trump as he spoke about the agreement on Friday. But he singled out the letter, saying it "did not go hand in hand with political and diplomatic courtesy."

"We haven't forgotten it," he added. "It would not be right for us to forget it."

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Dadouch and Khattab reported from Beirut. The Washington Post's Karen DeYoung in Washington contributed to this report.

This article was written by Kareem Fahim, Sarah Dadouch and Asser Khattab, reporters for The Washington Post.