Syrian rebels, backed by the Turkish military and US air cover, say they have taken the town of Jarablus from jihadists of so-called Islamic State.
The assault began at dawn when Turkish warplanes, tanks and special forces personnel crossed the nearby border.
Rebel commanders said most of the IS militants subsequently retreated.
Turkey says its intervention is targeting both IS fighters and a Syrian Kurdish-led alliance that is attempting to advance on Jarablus.
US Vice-President Joe Biden later confirmed the US had been flying air cover for the operation.
He also warned members of the Syrian Democratic Forces - the most effective opponents of IS on the ground in Syria - that they had to return to the east of the River Euphrates if they wanted to continue receiving its help.
"We have made it absolutely clear... that they must go back across the river," he said. "They cannot, will not, and under no circumstances get American support if they do not keep that commitment."
Making the highest-ranking visit to Ankara by a Western official since the failed Turkish coup on 15 July, Mr Biden also sought to dispel any doubts about America's solidarity with its Nato ally.
oe Biden is in Turkey to try and reset relations strained by the fallout from last month's failed coup attempt. Turks felt they did not get a clear message of support from Washington so Mr Biden is doing everything he can to send one - including deliberate comparisons to the trauma Americans suffered after the 9/11 attacks.
The Turkish government is also upset by America's alliance with Kurds in northern Syria. It believes they are linked to Turkish Kurds who are battling Ankara. The US finds the Syrian Kurds an effective force against Islamic State militants but the Turks fear the alliance is helping them gain territory for an autonomous zone along the Turkish border.
A US official travelling with Mr Biden admitted that some Kurdish fighters had pushed further north than they should have and said Turkey's offensive on IS in Jarablus was probably partly to create a buffer zone against any further Kurdish advance.
But he said the US had "put a lid" on any more such moves, creating a breathing space for the Jarablus operation, which the US supports and to which it is ready to contribute.
