ISTANBUL/BEIRUT, Feb 8- The Turkish army and allied Syrian rebels have captured the western outskirts of the Islamic State-held city of al-Bab, a rebel official and war monitor said on Wednesday, escalating their assault as the Syrian army also advanced on the city."With last night's assault, Islamic State's defenses have been broken through and the advance is now continuing," said a Turkmen Syrian rebel official, speaking from the Turkish city of Gaziantep.Syrian government forces have advanced to within a few kilometers (miles) of al-Bab, which is located 40 km (25 miles) northeast of Aleppo. The separate campaign by the Syrian army has raised the risk of a clash with the Turkish military.The Syrian Observatory for Human Rights, a UK-based organizsation that reports on the war, said the Turkish forces and their Free Syrian Army rebel allies had captured a hill on the western periphery of the city."We don't know if Daesh (Islamic State) will be able to recover it, or if it is in a state of collapse," Observatory Director Rami Abdulrahman said. The rebel official said Turkish reinforcements had been sent to the area a week ago.