U.S.-backed Syrian fighters waging a major new offensive against the Islamic State group vowed Thursday to cut off the last remaining access route to the outside world for the self-proclaimed caliphate, and won vital, if tacit, backing from Turkey.

The assault around the northern Syrian city of Manbij, backed by U.S.-led coalition airstrikes and a contingent of American special forces, aims to cut off the Islamic State's last 50-mile stretch of the Syrian-Turkish frontier by seizing territory west of the Euphrates River.

If successful, that would achieve a long-standing aim of Washington and amount to one of the biggest strategic defeats inflicted on the extremist group better known as ISIS or Daesh since it proclaimed its rule in Iraq and Syria two years ago.

"We confirm that this campaign will continue until the liberation of the last inch of the land of Manbij and its rural areas," said a statement read out on the banks of the Euphrates by Adnan Abu Amjad, a commander of a group called the Manbij Military Council, allied to the U.S.-backed Syria Democratic Forces. "Oh brave people of Manbij, our forces are coming to liberate you from the shackles of the Daesh terrorist torturers." 

Washington has been leading an international campaign of airstrikes against ISIS for two years in both Syria and Iraq. It has long been in search of reliable allies on the ground in Syria, where it also opposes the government of President Bashar Assad in a multi-sided civil war that has ground on for five years.

The SDF, set up last year, includes a powerful Syrian Kurdish militia and what Washington says are growing numbers of Arab forces that have been persuaded to join it. It has swept into villages west of the Euphrates since launching its offensive on Tuesday.

That advance comes as Iraqi army forces have separately begun an assault on the Iraqi city of Fallujah, 450 miles down the Euphrates at the opposite end of Islamic State's sprawling caliphate.

The Iraqi troops held their positions without advancing for a third straight day on Thursday, after pouring into the besieged city's southern outskirts on Monday. A Reuters reporting team in Saqlawiya, a village near Fallujah, saw Iraqi Shiite militia fighters in control of a complex of well-fortified trenches and tunnels captured from ISIS.

In other separate campaigns, Iraqi Kurds have also been advancing in villages near the northern Iraqi city of Mosul, and Russian-backed forces of the Syrian government have fought ISIS in other parts of Syria. The fighting amounts to some of the biggest pushes against the militants by their disparate enemies across a variety of fronts.

Turkish Support 

Washington hopes the assault near Manbij will be a turning point in the two-year conflict by choking off ISIS's last major link to the outside world. The militants have used the frontier for years to receive supplies and manpower, and more recently to send back fighters for attacks in Europe.

"We know that there is external plotting from Manbij city... against the homelands of Europe, Turkey, all good friends and allies of ours, and the United States as well," U.S. Defense Secretary Ash Carter said.

Of the SDF fighters, he said: "This is a capable force. They are doing all the things we can always do with able and motivated local forces to fight ISIL."

Washington's ultimate goals are to drive ISIS from its main bases: Raqqa in eastern Syria and Mosul in northern Iraq, to bring about the collapse of its control.

Kurdish fighters allied to Washington have already captured much of northeast Syria near the Turkish border, but their advance west of the Euphrates to close off the frontier once and for all was limited by strong opposition from U.S. ally Turkey, which considers the Kurdish YPG its enemies.

However, Turkey's President Recep Tayyip Erdogan signaled his apparent tacit support for the latest advance on Thursday, saying he had been informed that most of the fighters involved would be Arabs rather than Kurds.

Turkish military sources said Turkey had shelled Islamic State positions across the border at Azaz, west of where the advance was taking place, killing five militants. Medical charity Doctors Without Borders says 100,000 people are trapped near Azaz and in peril as the battle lines draw near.

A Kurdish source, speaking on condition of anonymity to Reuters in Beirut, predicted the Syrian militias would reach Manbij within days, after advancing to within six miles of the town.

It was too early to say how the battle for Manbij would go, the source said, but added that ISIS defenses on the west bank of the Euphrates River had collapsed at the start of the campaign.

However, Naser Haj Mansour, an adviser to the SDF general command, told Reuters Islamic State was still putting up a fight: "In general, the progress is at a good pace and performance, keeping in mind that Daesh still has the capability to fight."