ISIS terrorist in Syria
ISIS terrorist in Syria Reuters

The Islamic State (ISIS) group in Syria is about to lose access to Turkey's porous border, the Pentagon said Thursday, a vital step in blocking foreign fighters from replenishing the jihadists' thinning ranks.

According to Pentagon spokesman Captain Jeff Davis, ISIS now retains control of only about 25 kilometers (15 miles) of border with Turkey along an area to the east of the small Syrian town of Al-Rai.

"This is the only area with which ISIL has free communication with the outside world, where it touches a border," Davis said, using an alternative acronym for ISIS.

"This is about to be closed. We are very close to achieving this."

ISIS has lost ground in the key region as a result of Turkish operations that began last week.

Working with "moderate" and "vetted" Syrian rebel groups, Turkish forces swept into northern Syria -- assisted in part by U.S. air power -- and quickly took control of the border town Jarabulus.

The Turkish forces then began sweeping west along the border area.

Turkey's incursion into Syria has brought a new element to an already complex situation in the war-torn country, as Ankara has also targeted U.S.-backed Kurdish fighters.

The United States is working frantically to ensure fighters from the Kurdish People's Protection Units militia (YPG) are not further attacked by Turkey, and has encouraged them to move east of the Euphrates river and away from Jarabulus and another key city to the south, Manbij.

The Kurdish People's Protection Units, or YPG, are a key U.S. ally in the fight against ISIS.

Washington regards them as the most effective fighting force on the ground in Syria and has provided weapons and special forces military advisers.

The United States’ alliance with YPG has irked Turkish President Recep Tayyip Erdogan, as his country regards the YPG as a terror group and accuses it of carrying out attacks inside Turkey and being the Syrian branch of the Kurdistan Workers' Party (PKK).

AFP contributed to this report.