Turkey's prime minister's office said the Turkish military and the U.S.-backed coalition forces on Wednesday launched an operation to clear a Syrian border town from Islamic State militants.
The state-run Anadolu Agency said the operation began at 4 a.m. with Turkish artillery launching intense fire on Jarablus followed by Turkish warplanes bombing IS targets in the town.
It's not clear if any Turkish or Turkish-backed Syrian opposition forces have crossed the border.
The agency said the operation aims to clear Turkey's border of "terror organizations" and increase border security. It said the aim also is to "prioritize and support" Syria's territorial integrity.
The assault follows Turkish Foreign Minister Mevlet Cavusolgu pledge on Tuesday of "every kind" of support for operations against IS along a 100-kilometre (62-mile) stretch of Syrian frontier, putting the NATO member on track for a confrontation with U.S.-backed Kurdish fighters in Syria, who have been the most effective force against IS and who are eyeing the same territory.
Cavusolgu said Turkey would support twin operations stretching from the Syrian town of Afrin in the northwest, which is already controlled by Kurdish forces, to Jarablus, in the central north, which is held by the Islamic State group.
"It is important that the terror organizations are cleansed from the region," Cavusolgu said in a joint news conference with his Hungarian counterpart.
The Britain-based Syrian Observatory for Human Rights monitoring group said some 500 Syrian rebels were massed on the Turkish side of the border in preparation for an assault, including local fighters from Jarablus. One rebel at the border told the BBC the number was as high as 1,500 fighters.
The latest developments have thrust the town into the spotlight of the ongoing Syrian civil war. Jarablus, which lies on the western bank of the Euphrates River where it crosses from Turkey into Syria, is one of the last important IS-held towns standing between Kurdish-controlled areas in northern Syria.
Located 20 miles (33 kilometres) from the town of Manbij, which was liberated from IS by Kurdish-led forces earlier this month, taking control of Jarablus and the IS-held town of al-Bab to the south would be a significant step toward linking up border areas under Kurdish control east and west of the Euphrates River.
Turkey has increased security measures on its border with Syria, deploying tanks and armoured personnel carriers in recent days. On Tuesday, residents of the Turkish town of Karkamis, across the border from Jarablus, were told to evacuate after three mortars believed to be fired by IS militants landed there, Turkey's Dogan news agency said.
Turkey has vowed to fight IS militants at home and to "cleanse" the group from its borders after a weekend suicide bombing at a Kurdish wedding in southern Turkey killed at least 54 people, many of them children. Turkish officials have blamed IS for the attack.
Ankara is also concerned about the growing power of U.S.-backed Syrian Kurdish forces, who it says are linked to Kurdish groups waging an insurgency in southeastern Turkey.
The Kurdish-led group known as the Syria Democratic Forces, or SDF, recaptured Manbij from IS earlier this month, triggering concerns in Ankara that Kurdish forces would seize the entire border strip with Turkey. The U.S. says it has embedded some 300 special forces with the SDF, and British special forces have also been spotted advising the group.
Syrian activists, meanwhile, said that hundreds of Turkish-backed Syrian opposition fighters were gathered in the Turkish border area near Karkamis in preparation for an attack on Jarablus.
Nasser Haj Mansour, an SDF official on the Syrian side of the border, said the fighters gathering in Turkey include "terrorists" as well as Turkish special forces. He declined to comment on whether the SDF would send fighters to the town, but an SDF statement said the Syrian Kurdish force was "prepared to defend the country against any plans for a direct or indirect occupation."
The reports and rhetoric appeared to set up a confrontation between the SDF, the most effective U.S. proxy in Syria, and NATO ally Turkey.
A rebel commander affiliated with the SDF was killed shortly after broadcasting a statement announcing the formation of the so-called Jarablus Military Council and vowing to protect civilians in Jarablus from Turkish "aggression."
Abdel-Sattar al-Jader was shot by unknown gunmen late Monday, an hour after he accused Turkey of mobilizing fighters and "terrorists" for an assault on Jarablus. Al-Jader had pledged to resist Turkish efforts to take control of the city and warned Ankara against further aggression.
The Jarablus Military Council blamed the killing on Turkish security agents. There was no immediate comment from Turkey. Haj Mansour said two suspects were in custody but declined to comment on their identities.
The Kurds' outsized role in the Syrian civil war is a source of concern for the Syrian government as well. Fierce clashes erupted between the two sides over control of the northeastern province of Hasakeh last week, and Syrian warplanes bombed Kurdish positions for the first time, prompting the U.S. to scramble its jets to protect American troops in the area.
The Syrian government and the Kurds agreed on a cease-fire Tuesday, six days after the clashes erupted. The Kurdish Hawar News Agency said government forces agreed to withdraw from Hasakeh as part of the truce.
Syrian state media did not mention any withdrawal, saying only that the two sides had agreed to evacuate the wounded and exchange detainees. Government and Kurdish forces have shared control of Hasakeh since the early years of the Syrian war.
Meanwhile, the Syrian army and its allies intensified their attacks on militant positions around the so-called military college in the northern city of Aleppo.
A video obtained by The Associated Press showed air raids, shelling and firing on the complex, which was taken over by al-Qaida-affiliated fighters earlier this month. Plumes of smoke were seen billowing overhead.
Russia's Deputy Defence Minister Anatoly Antonov said in emailed comments on Tuesday that Russia and the Syrian government will announce "the first 48-hour humanitarian break in hostilities" in Aleppo as soon as they receive an official request from the U.N. envoy for Syria, Staffan de Mistura.