Witnesses reported clashes at the border and intense bombardments inside Syria
Turkey has launched a military operation to free a Syrian town held by Islamic State, according to state media and military sources.
As many as 20 tanks have crossed into Syria, according to reports, with between six and a dozen spotted by journalists from AFP and Reuters news agencies.
The numbers have not been confirmed by government officials but witnesses reported clashes at the border and intense bombardments inside Syria.
Hurriyet and Haber Turk television said efforts were underway to open a "passage" into the region as they head towards Jarabulus.
The Turkish tanks had been firing on Islamic State targets across the border in the Syrian town of Jarabulus since early Wednesday in the operation, named Euphrates Shield.
Turkish fighter jets - supported by US-led coalition aircraft - also struck Islamic State targets near the border town.
In retaliation, mortar rounds were fired from the IS-held Jarabulus, hitting the Turkish border town of Karkamis, which has been evacuated along with six other towns along the border.
Turkish counter-terrorism police also raided several homes in Istanbul, targeting Islamic State members in the early hours of Wednesday, detaining several people.
Interior Minister Efkan Ala said Turkey "cannot sit and watch", adding that it was Turkey's "legal right, it is within its authority" to act.
Turkey's President Recep Tayyip Erdogan confirmed the operation was to target Islamic State and the Kurdish PYD in Syria, adding that Turkey wanted to put an end to "frequent attacks".
Speaking in Ankara, he said that Turkey was seeking to "remove the shadow cast on the religion of Islam by Islamic State".
He added: "Nobody can see the Syrian issue independent from Turkey's internal affairs.
"The road to solving the terror problem passes through a solution to problems in Syria and in Iraq...Turkey will overcome threats originating from Syria."
As well as the threat from Islamic State, Turkey is also concerned about the growing influence of Kurdish groups in Syria's north.
It considers the PYD and its People's Protection Units (YPG) to be terrorist groups trying to carve out their own region, despite them also being an important part of the fight against IS.
Saleh Muslim, co-president of the PYD tweeted after Turkey's operation was confirmed that "Turkey is in Syrian quagmire, will be defeated as Daesh (will be)".
Redur Xelil, spokesman for the Syrian Kurdish YPG militia said the military operation was "blatant aggression in Syrian internal affairs".
On Saturday, 54 people were killed when a suicide bomber, though to be acting on the orders of Islamic State, detonated a bomb at a wedding party in Gaziantep.
Gaziantep lies around 60km north of the Syrian border and there are fears that jihadists are infiltrating the city.
Meanwhile, US Vice President Joe Biden has arrived in Ankara and will meet with senior officials.
Syria is expected to top the agenda, while it is reported that they will also discuss the Turkish request for the extradition of preacher Fethullah Gulen.