Russia's Foreign Ministry said in a statement on Wednesday Moscow was deeply worried by the escalation of tension on the Turkish-Syria border after Ankara sent military forces into Syrian territory.
Turkish special forces, tanks and jets backed by planes from the U.S.-led coalition launched their first co-ordinated offensive into Syria on Wednesday to try to drive Islamic State from the border and prevent further gains by Kurdish militia fighters.
A column of at least nine Turkish tanks crossed into northern Syria with Turkish-backed Syrian rebels to push Islamic State out of the border town of Jarablus, military sources said. A Reuters reporter at the border witnessed intense bombardments, with palls of black smoke rising around the town.
Testing time
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 a.m. (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.
U.S. 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 U.S. 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".
A senior U.S. official travelling with Biden said the United States 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-U.S. 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.
The Turkish army began firing artillery rounds into Jarablus at around 0100 GMT and Turkish and U.S. warplanes pounded Islamic State targets with air strikes.
It was the first time warplanes from Turkey have struck in Syria since November, when Turkey downed a Russian warplane near the border, and the first significant incursion by Turkish special forces since a brief operation to relocate the tomb of Suleyman Shah, a revered Ottoman figure, in February 2015.
Pre-empting Kurdish forces
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 U.S.-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.
Plumes of smoke rose from the hills around Jarablus, visible from the Turkish town of Karkamis across the border. The boom of artillery rounds was audible as advancing Turkish tanks fired.
Turkish military sources said the air strikes had hit 12 Islamic State targets, while artillery fire hit 70 targets.
"The aim of the operation is to ensure border security and Syria's territorial integrity while supporting the U.S.-led coalition against Islamic State," one military source said, adding work to open a passage for ground forces was under way.
Saleh Muslim, head of the Kurdish PYD, wrote in a tweet that Turkey was entering a "quagmire" in Syria and faced defeat there like Islamic State. Redur Xelil, spokesman for the Syrian Kurdish YPG militia, said the intervention was a "blatant aggression in Syrian internal affairs".
Kurdish groups control swathes of northern Syria where they established de facto autonomy since the start of the Syria war. The YPG, armed wing of the PYD, took control of most of Hasaka city on Tuesday, about 250 km (155 miles) east of Jarablus.
That growing Kurdish influence has alarmed Ankara, which is fighting its own insurgency with militants from the outlawed Kurdistan Workers Party (PKK), blamed by the government for an escalation of attacks in the southeast of Turkey.
The U.S.-backed Syria Democratic Forces alliance (SDF), which includes the YPG, captured the city of Manbij, just south of Jarablus, from Islamic State earlier this month.
The U.S. official acknowledged Turkey had not been happy about the Manbij operation but said Washington underlined the Kurds would pull back once the city was cleared and that they would not move north, addressing a major Turkish concern.
"We've put a lid on the Kurds moving north, or at least doing so if they want any support from us," he said.
Turkey had vowed on Monday to "completely cleanse" Islamic State militants from its border region after the Gaziantep bombing. Operation "Euphrates Shield" also comes after at least 10 mortar shells from Jarablus landed in Karkamis and the surrounding areas in recent days, forcing residents to flee.
Syrian rebels backed by Turkey had said they were in the final stages of preparing an assault from Turkish territory on Jarablus. A Syrian rebel with one of the Turkey-backed groups said about 1,500 fighters had gathered in Turkey to take part.
It is Turkey's first major military operation since the failed July coup by rogue soldiers who tried to overthrow Erdogan and the government, killing 240 people and triggering a purge of suspected coup supporters in the army and civil service.
Angered by a perceived lack of Western sympathy over the coup, Turkey chilled ties with Washington and the EU while ending a diplomatic row with Russia and proposing more military cooperation with Moscow in fighting Islamic State. Growing ties between Ankara and Moscow have worried Turkey's Western allies.