ISTANBUL — Turkey mounted its largest military effort yet in the Syrian conflict on Wednesday, sending tanks, warplanes, and special operations forces over the border and clearing the way for a Syrian rebel force to capture an important Islamic State stronghold in Syria.
The operation, a significant escalation of Turkey's role in the fight against the Islamic State, appeared to be making quick progress. By evening, rebel troops declared they had taken control of the town, Jarabulus, and its surroundings, with only one soldier killed, the semi-official Anadolu news agency reported. No Turkish troops died.
Numerous fighters posted photographs and videos of themselves online with the green, black, and white flag adopted by the Syrian opposition as they walked through what appeared to be empty streets, passing buildings still flying the flag of the Islamic State.
Earlier, Vice President Joe Biden signaled support for the goals of the operation — clearing Islamic State militants from their last major stronghold on the border and rolling back recent advances by Syrian Kurdish militias, which Turkey considers its primary enemy.
Biden had traveled to the Turkish capital, Ankara, on Wednesday to meet with President Recep Tayyip Erdogan at a time of high tensions between the two countries after the failed coup in Turkey last month. But the timing of the joint offensive and some strong words of support from Biden seemed to show an easing of the strains.
Speaking at a news conference after the meeting, Biden said the Syrian Kurds, an important US ally in the fight against the Islamic State, would have to meet a Turkish demand by withdrawing to the eastern side of the Euphrates River in northeastern Syria.
"We have made it clear to Kurdish forces that they must move back across the river," he said. "They cannot and will not get American support if they do not keep that commitment. Period."
It was an unusually sharp warning from the United States to the Kurdish-led forces.
Turkish officials indicated that the operation on Wednesday sought, in part, to warn Kurds working in Syria alongside US Special Operations forces against marching on Jarabulus. Cavusoglu warned that the Kurdish militias must move east of the Euphrates River, away from the Turkish border, and back to where they had long controlled a stretch of territory.
"If they fail to do so, we will do what is necessary," he said.
The operation started at 4 a.m., officials said, with Turkish and US warplanes pounding Islamic State positions in Jarabulus. The special operations troops entered Syria to clear a passage for a ground operation by Turkish-backed rebel groups, the state broadcaster TRT reported.