Turkish jets launched airstrikes against the northern Syrian town of Jarablus on Wednesday to clear it of Islamic State militants, state media reported saying it was a joint operation with the US-led coalition.
The military operation called "Euphrates Shield," which included shelling, was "aimed at clearing the district of Jarablus of the province of Aleppo from the terrorist organization," a government statement said.
The state-run Anadolu news agency said one of the goals of the mission, which began before dawn, was to support the territorial integrity of Syria.
The operation comes on the day of a planned visit to Ankara by US Vice President Joe Biden, the most senior official from Washington to visit Turkey since the failed coup of July 15.
The private Dogan news agency said the air force hit 12 targets inside Syria. Anadolu also reported on a series of police raids in Istanbul aimed at detaining Islamic State suspects.
The leader of the main Syrian Kurdish party denounced the Turkish military operation inside Syria.
"Turkey is in Syrian Quagmire. will be defeated as Daish," Salih Muslim, head of the Democratic Union Party (PYD) tweeted, using an Arabic acronym for Islamic State.
The PYD and its armed wing, the People's Protection Units (YPG), have been allied with the US in the war against Islamic State.
Turkey appears keen to take Jarablus before Kurdish-led forces move on the town.
Kurdish forces, the main US ally inside Syria, this month seized nearby Minbij from Islamic State.
Islamic State has controlled territory along Turkey's border with Syria since 2013. Jarablus itself fell under its control that year, after Islamic State ousted Syrian rebel factions.
In late 2014, US airstrikes and Kurdish ground forces began operations to push Islamic State off the border, working to cut off its supply routes and flows of foreign fighters from over the porous Turkish border.
Islamic State currently only controls a small stretch of territory, between Jarablus and al-Rai, about 50 kilometres away. The Kurds now hold most of the land Islamic State ceded in the area.
Turkey has reportedly opposed the Kurds moving on Jarablus, fearing their widening presence on its border.
On Monday, Turkey began shelling both Islamic State and Syrian Kurds in areas just across the border. Islamic State retaliated the next day and about a dozen rockets were fired into Turkey. Residents in some border districts were evacuated.
Turkey has been repeatedly hit by Islamic State attacks since last year. On Saturday, a suicide bomber killed 54 people, mostly children, at a Kurdish wedding in Gaziantep, in the south. Authorities blame Islamic State.
Syrian rebel forces told dpa this week that Turkey was keen to move them from north-western Syria to Jarablus to engage in ground battles there. The rebels have no direct front line with the town.
Both Turkey and Syrian rebel forces, including Islamist factions, oppose Kurdish efforts to carve out autonomous zones for themselves in the north of the country on land taken from Islamic State.
The Kurds, a minority group, have long complained of discrimination in Syria and view federalism as a solution to the country's long running civil war.
Turkey and Russia recently mended fences after relations deteriorated in November, when Ankara downed a Russian fighter jet over the Syrian border.
Turkey had since refrained from flying missions inside Syria, instead using shells to carry out cross border attacks.
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The Syrian government says the Turkish operation in northern Syria is a "flagrant breach" of the country's sovereignty.
Turkish tanks crossed the border into Jarablus in northern Syria, state broadcaster TRT reported Wednesday citing military sources, while Syrian rebels forces told dpa they are also crossing into the Islamic State-held town from Turkey.
Syrian Kurds condemn the Turkish incursion into Islamic State-held Jarablus, calling it a "declaration of war."