DAMASCUS, Feb. 24 (Xinhua) -- At least 30 people were killed on Friday when a suicide bombing believed to be carried out by an Islamic State (IS) fighter targeted rebel position near Syria's northern city of al-Bab, a monitor group reported.

The blast rocked the town of Susian west of al-Bab, in what was said to be a suicide bombing targeting Turkey-backed rebels, who, along with Turkish forces, captured al-Bab a day earlier after dislodging the Islamic State (IS) out of that key area near the Turkish borders.

The Syrian Observatory for Human Rights said most of the causalities were rebels. Other activists placed the death toll at 50.

A day earlier, the IS group announced its withdrawal from al-Bab, which has been taken by the Turkish forces and allied Syrian rebels on Thursday, according to pan-Arab al-Mayadeen TV.

The withdrawal of the IS from al-Bab, its largest stronghold in northern Syria near the Turkish borders, came after 100 days of battles against the Turkish forces and allied Syrian rebels fighting in an operation called the Euphrates Shield.

The IS claimed it had killed 400 Turkish soldiers and rebel fighters.

For the Turks, capturing the northern part of the city cut the way in the face of the growing Kurdish influence in northern Syria, a red line drawn by Turkey.

The Syrian army succeeded recently to besiege al-Bab from its southern edge, a move to prevent IS fighters to withdraw toward other stronghold in the eastern province of Deir al-Zour, or the northern city of al-Raqqa, the de facto capital of the terror-designated group.

Observers said the attacks on al-Bab were coordinated between the Russians and the Turks.