A U.S.-backed alliance of Syrian fighters including the Kurdish YPG militia captured a town held by the Islamic State group in Syria's northeast Friday, the Syrian Observatory for Human Rights monitoring group said.

The capture of al-Shadadi in Hasaka province came three days after the beginning of an offensive against the terrorist organization, aka ISIS, in the area by the Syrian Democratic Forces backed by U.S.-led airstrikes, and likely will help isolate Raqqa, the jihadists' de facto capital in Syria.

There was no immediate comment from the Syrian Democratic Forces.

Rami Abdulrahman, director of the Britain-based Syrian Observatory for Human Rights, said al-Shadadi's capture was a "psychological blow" to ISIS. He said it still controlled dozens of villages in the area but that the SDF had taken many more villages and farmland in the past few days.

On Thursday the SDF announced it had launched an operation earlier in the week to seize al-Shadadi from ISIS.

The U.S.-backed alliance, formed in October, includes the powerful YPG, which has proved the most effective partner against ISIS on the ground in Syria for a Washington-led air campaign against the group there and in neighboring Iraq.

The SDF, which also includes Arab fighting groups such as Jaysh al-Thuwwar, captured areas of Hasaka from ISIS late last year.

The YPG took swaths of territory from the group in 2015. Most of Hasaka province is under Kurdish control.