Thousands of US-backed fighters opened a major new front in Syria's war, launching an offensive to drive Islamic State out of a swathe of northern Syria it uses as a logistics base, and were reported on Wednesday to be making rapid progress.
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 US military official told Reuters.
A small number of US special operations forces will support the push on the ground, acting as advisers 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.
Driving Islamic State from its last remaining foothold at the Turkish border has been a top priority of the US-led campaign against the group. The group controls around 50 miles of the frontier stretching west from Jarablus.
The Syrian Observatory for Human Rights said US-led air strikes in support of the ground operation killed 15 civilians, including three children, near Manbij in the last 24 hours. The Observatory's reporting is based on an activist network in Syria.
It said the Syria Democratic Forces (SDF) alliance, the force that is conducting the assault to capture the tract of land known as the Manbij pocket, had taken 16 villages and were 9 miles from Manbij town itself.
The US officials said the operation would 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.
This is seen as important to NATO member Turkey, which has opposed any further expansion of Syrian Kurdish sway at the frontier.
Ankara sees the Syrian Kurdish YPG fighters, who already control an uninterrupted 250 mile stretch of the border, to be terrorists and has been enraged by US backing for the militia in its battle with Islamic State in Syria.
However, the Observatory said the Kurdish YPG militia made up the majority of the fighters taking part in the SDF assault.
A US official said Turkey supported the offensive. SDF and YPG officials could not immediately be reached for comment.
The US officials told Reuters the YPG would only fight to help clear Islamic State from the area around Manbij. Syrian Arab fighters would be the ones to stabilize and secure it once Islamic State is gone, according to the operational plans.
"After they take Manbij, the agreement is the YPG will not be staying ... So you'll have Syrian Arabs occupying traditional Syrian Arab land," the official said.
In Ankara, a Turkish military source said Turkey was not contributing to the operation. Ankara had been told by Washington about the push but could not back it because of the involvement of Kurdish YPG fighters and because it was beyond the range of artillery stationed in Turkey, the source said.
The operation is happening ahead of an eventual push by the US-backed Syrian forces toward the city of Raqqa, the Islamic State's defacto capital in Syria and the prime objective in Syria for US military planners.
The US military official said depriving Islamic State of the Manbij pocket would help further isolate the militants and further undermine their ability to funnel supplies to Raqqa.
US President Barack Obama has authorized about 300 US special operations forces to operate on the ground from secret locations inside Syria to help coordinate with local forces to battle Islamic State there.
In a reminder of the risks, one US service member was injured north of Raqqa over the weekend, the Pentagon said.
Syrian Kurdish groups have established their own government in northeastern Syria since 2011. Capturing the last remaining Islamic State foothold at the Turkish border would help them to link up with the area of Afrin, which is controlled by the same Kurdish groups in northwestern Syria.
The YPG has been the most effective ally on the ground for US-led air strikes against IS, and last year captured large areas from it in Hasaka province.
The SDF alliance, including some Arab militias, was formed in October, and has led the campaign against IS with US support.
The SDF last week began attacks against Islamic State in areas north of Raqqa, but says the city was not a target of that operation.