DAMASCUS, March 26 (Xinhua) -- At least seven fighters of the U.S.-backed Syrian Democratic Forces (SDF) were killed overnight by a surprise attack of Islamic State (IS) sleeper cells in northern Syria, a war monitor reported Tuesday.

The IS operatives launched their surprise attack against the SDF at a checkpoint between the city of Manbij, in the northeastern countryside of Aleppo province, and the provincial city of Aleppo, according to the Syrian Observatory for Human Rights.

The Britain-based watchdog said three other SDF fighters were wounded and search is underway for the perpetrators.

The attack comes days after the SDF rounded up tens of sleeper cells of IS in Manbij.

The observatory said that between 4,000 and 5,000 IS sleeper cells are present in areas controlled by the SDF in northern Syria.

On March 23, the SDF declared the elimination of the IS organization in eastern Syria and the end of the self-declared "caliphate" in the war-torn country.