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.