A dozen Turkish tanks and other vehicles have rolled across the Syrian border after heavy shelling of an area held by so-called Islamic State (IS).
Military sources told Turkish media 70 targets in the Jarablus area had been destroyed by artillery and rocket strikes, and 12 by air strikes.
Turkish-backed Syrian rebels are accompanying the Turkish advance.
President Recep Tayyip Erdogan said the operation was aimed against both IS and Kurdish fighters.
Turkey shelled Syrian Kurdish forces in the region this week, determined not to let them fill the vacuum if IS leaves, the BBC’s Mark Lowen reports from Gaziantep, near the Syrian border.
The concern in Ankara is that the Kurds could create an autonomous area close to the border which might foster Kurdish separatism within Turkey itself, our correspondent says.
In another development, counter-terror police in Turkey’s main city, Istanbul, launched dawn raids targeting IS suspects across the city.
US Vice-President Joe Biden arrived in Turkey on Wednesday in the highest-ranking visit by a Western official since the failed coup on 15 July.
The tanks were followed by pick-up trucks believed to be carrying Turkish-backed Syrian rebels from the Free Syrian Army (FSA).
The FSA said its fighters had taken control of the village of Hjeliya near Jarablus. The operation was moving slowly, it said, because of mines planted by IS in the area.
“We’re going on until full liberation of Jarablus city from terrorist ISIL [IS],” said one of its commanders, Captain Abo Ibraheem.
President Erdogan announced in a speech in Ankara: “At 04:00 [01:00 GMT] our forces began an operation against the Daesh [IS] and PYD [Kurdish Democratic Union Party] terror groups.”
The offensive was aimed at “putting an end” to problems on the border, he said.
The Turkish town of Karkamis – just across the border from Jarablus – was evacuated as a precaution following earlier IS mortar attacks.
Turkey has vowed to “completely cleanse” IS from its border region, blaming the group for a bomb attack on a wedding that killed at least 54 people in Gaziantep on Saturday.
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 of last year.
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.
‘A buffer against the Kurds’
An unnamed senior US official in Washington told BBC News before the start of the Turkish operation that it was “partly to create a buffer against the possibility of the Kurds moving forward”.
Fighters from the Syrian Kurd YPG militia – the military wing of the PYD – led the battle to drive IS out of the strategic crossroads town of Manbij earlier 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 IS.
Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), a Turkish-Kurdish rebel group fighting for autonomy since the 1980s, but the YPG is backed by the US as one of the most effective forces battling IS.
On Tuesday the YPG took control of most of the north-eastern Syrian city of Hassakeh after a truce reportedly brokered by Russia with Syrian government forces.
President Erdogan said he would press Vice-President Biden for the extradition of US-based cleric Fethullah Gulen, whom he blames for the coup attempt.
Turkish media round on Biden
Several pro-government papers accuse Washington of dragging its heels after Turkey’s call for the extradition of US-based Turkish cleric Fethullah Gulen. “Gulen’s protector” says a headline in Aksam, which notes his “late” visit in the wake of last month’s coup attempt.
Gunes newspaper says pointedly that “the Turkish government and 79 million Turks are waiting for concrete steps” from him, adding that “Biden will be… asked to behave like an ally”.
The pro-government Star agrees that Mr Biden will have to explain US support for the “Fetullah Gulen Terrorist Organisation” (FETO). Karar says the visit will open a new chapter in bilateral ties but says Washington must show its support for Turkey.
Pro-government daily Milliyet publishes a column by Mr Biden himself where he praises Turks for thwarting the coup attempt. He also says that claims that Gulen’s extradition is simply a political matter are “damaging” for bilateral ties.
Pinar Sevinclidir, BBC Monitoring’s Turkish media analyst