Turkey has sent tanks and other vehicles across the Syrian border after heavy shelling of an area held by ISIS.
Military sources told Turkish media 70 targets in the Jarablus area had been destroyed by artillery and rocket strikes, and 12 by air strikes.
Syrian rebels who are following the advance say they have entered the town of Jarablus itself.
The operation is aimed against both ISIS and Kurdish fighters.
Turkey shelled Syrian Kurdish forces in the region this week, determined not to let them fill the vacuum if ISIS leaves.
US Vice-President Joe Biden warned Kurdish forces in Syria they would lose US support if they advanced west of the River Euphrates.
Making the highest-ranking visit to Ankara by a Western official since the failed Turkish coup on July 15, Joe Biden also sought to dispel any doubts about America’s solidarity with its NATO ally.
Turkey’s President Recep Tayyip Erdogan announced in a speech in Ankara: “At 04:00 [01:00 GMT] our forces began an operation against the Daesh [ISIS] and PYD [Kurdish Democratic Union Party] terror groups.”
Operation Euphrates Shield was aimed at “putting an end” to problems on the border, he said.
Between 9 and 12 tanks crossed the border, followed by pick-up trucks believed to be carrying Turkish-backed Syrian rebels from the Free Syrian Army (FSA).
The FSA said progress was slow because of mines planted by ISIS fighters in the area. There were no immediate reports of fighting on the way in.
Turkey has vowed to “completely cleanse” ISIS from its border region, blaming the group for a bomb attack on a wedding that killed at least 54 people in Gaziantep on August 20.
This is Turkey’s first known ground incursion into Syria since a brief operation to relocate the tomb of Suleyman Shah, a revered Ottoman figure, in February 2015.
The air strikes are Turkey’s first inside Syria since the downing of a Russian jet in November. Moscow and Ankara only mended ties in June after punitive Russian sanctions.
Fighters from the Syrian Kurd YPG militia – the military wing of the PYD – led the battle to drive ISIS out of the strategic crossroads town of Manbij this month.
Responding to news of the Turkish advance, PYD leader Saleh Moslem tweeted that Turkey was now in the “Syrian quagmire” and would be defeated like ISIS.