Turkish-backed Syrian rebels entered the Islamic State-held town of Jarablus in northern Syria on Wednesday in an operation supported by Turkish tanks, special forces units and US-led coalition warplanes, rebel sources said.

A rebel commander with the Failaq Al Sham group, who requested anonymity, told Reuters that most of the Islamic State fighters in Jarablus had pulled out, some of them surrendering. Another commander estimated up to 50 percent of the town was now under the control of the Turkish-backed rebels.

"Daesh (Islamic State) fighters have withdrawn from several villages on the outskirts of Jarablus and are heading south towards the city of al Bab," the Failaq al Sham commander said.

A column of at least nine Turkish tanks crossed into northern Syria with Turkish-backed Syrian rebels in the first co-ordinated offensive into Syria. A Reuters reporter at the border witnessed intense bombardments, with palls of black smoke rising around the town.

President Tayyip Erdogan said the operation was targeting Islamic State and the Kurdish PYD party, whose gains in northern Syria have alarmed Turkey.

Ankara views the PYD as an extension of Kurdish militants fighting an insurgency on its own soil, putting it at odds with Washington, which sees the group as an ally in the fight against Islamic State.

"This morning at 4 am (0100 GMT) an operation started in northern Syria against terror groups which
constantly threaten our country, like Daesh (Islamic State) and the PYD," Erdogan said in a speech in
Ankara.

US Vice President Joe Biden arrived in Turkey, which has Nato's second biggest armed forces, hours
after operations began on a pre-planned trip. Biden is the most senior US official to visit since a failed July 15 coup shook confidence in Turkey's ability to step up the fight against Islamic State.

"Euphrates Shield", named after the river running nearby, is Turkey's first major military operation since the abortive coup. A military source said the Turkish-backed rebels had seized control of four villages as they pushed towards Jarablus.

The offensive by Turkey comes four days after a suicide bomber suspected of links to Islamic State killed 54 people at a wedding in the southeastern city of Gaziantep.

Syria's foreign ministry condemned what it said was a breach of its sovereignty and accused Ankara of launching the incursion to replace Islamic State with "other terrorist groups".

TESTING TIME

A senior US official travelling with Biden said the US wanted to help Turkey to get Islamic State
away from the border, and was providing air cover and "synching up" with the Turks on their plans for
Jarablus. The shelling was hitting Islamic State, not Kurdish forces, he said.

Biden's visit comes at a testing time for Turkish-US relations. Turkey says the failed putsch was staged by Muslim cleric Fethullah Gulen, who has lived in self-imposed exile in Pennsylvania for the past 17 years.

Erdogan wants Gulen extradited but Washington says it needs clear evidence of his alleged involvement, sparking an outpouring of anti-Americanism from Turkey's pro-government media. Gulen denies any involvement in the attempted coup.

Turkey and the United States hope that by removing Islamic State from the border, they can deprive it of a smuggling route which long saw its ranks swollen with foreign fighters and its coffers boosted by illicit trade.

But for Turkey, it also preempts any attempt by Syrian Kurdish militia fighters, who play a critical part of the US-backed campaign against Islamic State, to take Jarablus.

Kurdish fighters have captured large areas of territory since the start of the Syrian conflict in 2011, and
Ankara has long declared the Euphrates river, which runs just east of Jarablus, a red line which it does not want them to cross.

Foreign Minister Mevlut Cavusoglu said Kurdish fighters must return east of the Euphrates or Turkey
would "do what is necessary". He said the operation was a turning point and would accelerate removing Islamic State from Syria's Aleppo region. - Reuters