The Turkish army killed five ISIS militants in Syria in cross-border shelling, Turkish military sources said on Thursday.

The army targeted positions west of where an offensive on militants was launched by Syrian fighters with U.S. backing.

Thousands of Syrian rebels supported by a small U.S. special operations team launched a major offensive on Tuesday to drive ISIS from the “Manbij pocket” near the Turkish border, which ISIS has used as a logistics hub.

The operation, which began on Tuesday after weeks of quiet preparations, aims to choke off the group’s access to Syrian land along the Turkish border that the militants have long used to move foreign fighters back and forth to Europe.

“It’s significant in that it’s their last remaining funnel” to Europe, a U.S. military official told Reuters.

A small number of U.S. special operations forces will support the push on the ground to recapture the territory, acting as advisers and staying back from the front lines, the officials said, speaking on condition of anonymity to discuss military planning.

“They’ll be as close as they need to be for the (Syrian fighters) to complete the operation. But they will not engage in direct combat,” the official said.

Washington informed Ankara of the Manbij operation, but it was beyond the range of Turkish artillery and Turkey would not back a campaign in which Syrian Kurdish fighters played a role, another Turkish military source said on Wednesday.

Turkish border guards fired artillery at two ISIS positions near the Syrian town of Azaz on Wednesday, the Turkish military sources said, west of the U.S.-backed operation and directly south of the Turkish border town of Kilis, which has been repeatedly hit by ISIS rockets.

Turkey objects to the United States’ backing of the Kurdish YPG militia against ISIS in Syria because it considers the group to have deep ties to the Kurdistan Workers Party (PKK) militant group waging an insurgency in the Turkish southeast.

Prime Minister Binali Yildirim on Wednesday said weapons given to the YPG militia in Syria were making their way to PKK militants in Turkey.

“The side elements of the terrorist organization (PKK) in Syria and Iraq are procuring these in the name of the struggle against terrorism and transfer them to the terrorist organizations in Turkey. This is unacceptable,” he said.

Turkey is trying to stamp out the PKK insurgency after it re-ignited last July when a two-year ceasefire collapsed. The United States and Turkey both list the PKK as a terrorist group.

Turkish jets struck PKK camps in northern Iraq early on Thursday and destroyed seven rebel targets the Metina area, the military sources also said.

The PKK is mainly based in mountainous northern Iraq, and Turkey regularly enters Iraqi airspace to strike their camps and weapon stores, despite objections from Baghdad and the semi-autonomous Kurdistan Regional Government.