Heavy shelling of positions held by the so-called Islamic State has marked the start of a Turkish offensive to drive the militants away from the Syrian border.
Artillery and rocket launchers rained 224 rounds on the Jarablus area, the military said, as Turkish jets bombed their first targets in Syria this year.
Tanks could later be seen moving up to the border and opening fire.
Turkish special forces are already inside Syria as part of the operation to clear IS out of Jarablus.
If Jarablus falls, the jihadists will be pushed back from the Turkish border.
Turkey has also hit Syrian Kurdish forces in the region, determined not to let them fill the vacuum if IS leaves, BBC correspondent 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, the correspondent says.
In another development, counter-terror police in Turkey's main city, Istanbul, launched dawn raids targeting IS suspects across the city, Turley's Dogan news agency reports.
Turkey has vowed to "completely cleanse" IS from its border region.
"The Turkish Armed Forces and the International Coalition Air Forces have launched a military operation aimed at clearing the district of Jarablus of the province of Aleppo from the terrorist organisation Daesh [IS]," said a statement from the Turkish prime minister's office.
A Turkish military source told Reuters: "The aim of the operation is to ensure border security and Syria's territorial integrity while supporting the US-led coalition against Islamic State."
Turkey blames IS for a bomb attack on a wedding that killed at least 54 people in Gaziantep on Saturday.
A force of 1500 Turkish-backed Syrian rebels is said to be waiting in the city to take part in the Jarablus operation.
Shelling began about 4am local time and US jets flew missions as well as Turkish, military sources told Reuters.
The 224 rounds were fired at 63 targets in the space of one hour and 45 minutes, and air strikes began just after 6am, Turkish media report.
The Turkish town of Karkamis - just across the border from Jarablus - was evacuated as a precaution following earlier IS mortar attacks.
Turkey's air strikes are its first inside Syria since the downing of a Russian jet in November. Moscow and Ankara only mended ties in June after punitive Russian sanctions.
This is also 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.
In recent days the Turkish military also shelled positions belonging to the Kurdish YPG militia, apparently to deter them from taking Jarablus themselves.
Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), a Turkish-Kurdish rebel group fighting for autonomy since the 1980s.
However the YPG is backed by the US in the battle against IS and has been gaining territory in northern Syria.
On Tuesday it took control of most of the north-eastern Syrian town of Hassakeh, one of the few northern areas where Syrian government forces still have a hold.