Istanbul (dpa) - At least four Turkish soldiers have been killed in heavy fighting in northern Syria, according to state news agency Anadolu.Fifteen soldiers were injured in military operations against Islamist militants near the town of al-Bab, which is controlled by the Islamic State extremist group, the agency reported on Wednesday, citing military sources.Turkey launched a ground offensive in northern parts of the war-torn nation in August in cooperation with Syrian anti-government forces, attacking both Islamic State and US-backed Kurdish forces there.More than 40 Islamic State fighters were "neutralized," Anadolu reported on Wednesday, without specifying whether they had been killed, injured or captured.Islamic State for its part said it had carried out a suicide attack west of al-Bab against Turkish soldiers and Syrian rebels. The group gave no further details via the affiliated Amaq news agency.The Turkish air force has carried out 24 strikes in the al-Bab region. The Turkish and rebel offensive has brought the road between Aleppo and al-Bab under control, Anadolu reported.Syrian opposition rebels, backed by Turkish air power and shelling, on Saturday entered al-Bab itself, the Syrian Observatory for Human Rights reported.The progress by the rebels and Turkish allies appears aimed at stopping the forces of Syrian President Bashar al-Assad from themselves taking over al-Bab.