ANKARA, Turkey — Turkish artillery shelled Islamic State targets across the border in Syria for the second consecutive day on Tuesday, a senior Turkish official said, amid reports that Turkey-backed Syrian rebels are preparing an offensive against an IS-held border town.

The latest developments have thrust the town of Jarablus onto center stage in the ongoing Syrian civil war, putting U.S.-backed Kurdish forces, who have been the most effective force against IS in northern Syria, on track for a confrontation with NATO ally Turkey over control of the town.

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 33 kilometers (20 miles) from the town of Manbij, which was liberated from IS by Kurdish-led forces earlier this month.

The Ankara official said the Turkish shelling came after mortar rounds, believed to have been fired by the militants from Jarablus, landed on Turkish territory. The official, who spoke on condition of anonymity in line with government regulations, did not provide further details.

Turkey has increased security measures at its border opposite IS-held Jarablus, deploying tanks and armored personnel carriers in recent days. Turkey has also 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 that 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.

A Kurdish-led group known as the Syria Democratic Forces earlier this month liberated Manbij, triggering concerns in Ankara that they would seize the entire border strip with Turkey. The U.S. says it has embedded some 300 special forces with the SDF. French and British special forces have also been spotted advising the SDF.

Syrian activists meanwhile say that hundreds of Turkish-backed Syrian opposition fighters are gathered in the Turkish border area of 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 a statement from the SDF said “we are prepared to defend the country against any plans for a direct or indirect occupation.”