Turkey's artillery has shelled Islamic State group targets across the border in Syria for the second consecutive day.
It comes amid reports that Turkey-backed Syrian rebels were preparing an offensive against an IS-held border town in northern Syria.
The latest developments have thrust the town of Jarablus into centre stage in the ongoing Syrian civil war.
The Turkish shelling came after mortar rounds, believed to have been fired by the militants from Jarablus, landed on Turkish territory.
Turkey has increased security measures at its border opposite the Islamic State-held Jarablus, deploying tanks and armoured personnel carriers in recent days.
Syrian activists have said that hundreds of Turkish-backed Syrian opposition fighters are gathered in the Turkish border area of Karkamis in preparation for an attack on Jarablus.
Ankara is concerned about the growing power of US-backed Syrian Kurdish forces, who it says are linked to Kurdish groups waging an insurgency in south-eastern Turkey.
In Syria, a Kurdish-led group known as the Syria Democratic Forces earlier this month liberated the nearby town of Manbij from the Islamic State group, triggering concerns in Ankara that they would seize the entire border strip with Turkey.
The US has embedded some 300 special forces operators with the SDF, according to the government. French and British special forces operators as well have been spotted advising the SDF.
Jarablus is a vital supply line and the last border point that directly connects the Islamic State group with Turkey and the outside world, and separates Kurdish-controlled areas in northern Syria.
The town is 20 miles from Manbij, which is now under SDF control.
An official with the US-backed Syrian Democratic Forces on the Syrian side of the border claimed the fighters allegedly gathering in Turkey include "terrorists" as well as Turkish special forces operators.
"We confirm we are prepared to defend the country against any plans for a direct or indirect occupation," a statement from the SDF said.
The developments put the US's only clear proxy in Syria, the SDF, and its Nato ally, Turkey, on track for a confrontation over Jarablus, though the SDF official, Nasser Haj Mansour, would not comment on whether the SDF would send fighters to the town.
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 on Monday, an hour after reading the statement posted online in which he also accused Turkey of mobilising fighters and "terrorists" through the border toward Jarablus. Al-Jader pledged to fight Turkish efforts to take control of the city and warned Ankara against further aggression. The SDF also issued a statement in which it declared its full support for the council.
The Jarablus Military Council, in a statement issued after al-Jader's death, accused Turkish security agents of being behind the killing.
Haj Mansour said two suspects were in custody but would make no comment about their identities.
The Hurriyet newspaper and others said the mortar rounds hit the town of Karkamis, in Turkey's Gaziantep province, on Tuesday. One of them exploded in the garden of a house, but no one was hurt.
Anadolu Agency said Turkish artillery fired 40 rounds against IS targets in retaliation, after three rockets fired from Syria landed into an empty field in the town of Kilis. The report says the Turkish Armed Forces "covered the area with fire" without providing any details. No one was hurt by the rockets.
On Monday, Turkish artillery attacked a US-backed Syrian Kurdish militia as well as IS positions in Syria.