BEIRUT (Reuters) – Islamic State fighters have evacuated their families from a Syrian town at the Turkish border near a city that they recently lost to U.S.-backed militias, a monitor group said on Friday, a sign they may be preparing to face an attack there.

Last week’s capture by the Syrian Democratic Forces (SDF) of Manbij, 25 miles (40km) to their south, has left Islamic State fighters in Jarablus in danger of being cut off from the militant group’s main territorial possessions.

The town is located at the eastern edge of an Islamic State salient stretching 33 miles (55km) along the Turkish border, and could be encircled by any SDF thrust northwards from its positions further to the west. SDF positions on the Euphrates already look directly across to Jarablus on the opposing bank.

More than 50 families of Islamic State fighters and leaders arrived in the group’s stronghold of Raqqa from Jarablus and the larger town of al-Bab, between Manbij and Aleppo, the Syrian Observatory for Human Rights, a U.K.-based war monitor, said.

Separately, a senior Syrian rebel source told Reuters that Islamic State was moving personnel out of Jarablus.

The SDF have not yet declared what their next target will be after capturing Manbij. A successful advance north could cut Islamic State off from the Turkish border, while a thrust west could threaten al-Bab, an important Islamic State stronghold.

After Manbij fell to the SDF, some local fighters announced they had established a military council for al-Bab, signaling they believed an assault on Islamic State in the town would soon take place. The SDF denied having any links to the council.

The U.S.-backed SDF is made up of both Kurdish fighters, including the YPG militia, and local Arab armed groups. It has denied any links to a military council established last week.

(Writing by Tom Perry)