Thousands of US-backed fighters in Syria are launching an offensive to capture from Islamic State a crucial swathe of northern Syria known as the Manbij, US officials say.
The operation, which only just started to get underway 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 US military official said.
A small number of US special operations 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 US-led coalition air strikes as well as from ground-based firing positions across the border in Turkey.
Perhaps essential for NATO ally Turkey, the operation will be overwhelmingly comprised of Syrian Arabs instead of forces with the Kurdish YPG militia, who will only represent about a fifth or a sixth of the overall force, the officials said.
Ankara considers the Syrian Kurdish YPG fighters to be terrorists and has been enraged by US backing for the militia in its battle with Islamic State in Syria.