U.S. officials have disclosed that thousands of U.S.-backed fighters in Syria are launching an offensive to capture from Islamic State a crucial swathe of northern Syria known as the Manbij pocket following weeks of quiet preparations.

The operation, which only just started to get underway on Tuesday and could take weeks to complete, aims to choke off Islamic State’s access to Syrian territory along the Turkish border that militants have long used as a logistics base for moving 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 said.

“A small number of U.S. special operation forces will support the offensive on the ground, acting as advisors and staying some distance 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.

The operation will also count on support from U.S.-led coalition air strikes as well as from ground-based firing positions across the border in Turkey.