A massive operation involving thousands of US-backed troops of Kurds and Arabs in Syria is under way against Islamic State (Isis). The US had been preparing for the offensive for several weeks to capture Manbij area along the Syria-Turkey border from Isis, US officials told Reuters.

The Daesh militants have been using the Manbij border strip as a key entry point for foreign fighters to Europe, and the passage is said to be the last remaining channel for Isis to Europe.

The US special operations forces will be deployed in small numbers and they will not engage in active combat. They will only be supporting the offensive on the ground by acting as advisers to the Syrian fighters. However, the US will support the troops in launching coalition air strikes and be involved in ground-based firing positions across the border in Turkey.

"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," an American official told Reuters on condition of anonymity.

The operation will mostly comprise of Syrian Arabs, while the Kurdish YPG (People's Protection Unit) militia will represent a small fraction of the overall troops. The YPG militia is the dominant group in the Syria Democratic Forces (SDF) that has been working closely with the US to take territorial control from Isis.

Turkey has been seeing the Syrian Kurdish YPG fighters as terrorists, and Washington's support to the fighters has left its Nato ally Ankara furious. As a result, Turkey had even opposed the idea of YPG fighters taking control of Manbij pocket. Besides, the Kurdish forces' unhindered control of a 400 km (250 miles) stretch on the border leaves Turkey worried.

US officials have clarified that YPG will only fight to throw Isis out of the area around Manbij, and that the Syrian Arab fighters will have to step up and retain authority once the area is captured. Turkey has supported the offensive in a bid to isolate Isis further from Syria and challenge their access to funnel supplies to Raqqa.

Meanwhile, the Pentagon announced that two American soldiers were injured in separate attacks by Isis in Iraq and Syria, the first time US servicemen have been hurt in the Arab nation since military advisers were deployed there by the end of last year.

Isis: Kurdish militias battle forces in north-east Syria IBTimes UK