A Syrian air force raid killed at least 59 civilians at a market in a town held by Islamic State militants in northern Syria on Saturday, according to a group monitoring the war.The strike on the town of al-Bab, northeast of the city of Aleppo, used "barrel bombs" - steel drums full of shrapnel and explosives, the Syrian Observatory for Human Rights said. Syrian President Bashar Assad has denied the air force uses such devices.Dozens of people were wounded in the attack, according to the monitoring group, which gathers information from sources on the ground. The town has been the focus of several heavy bombardments by the military since late last yearA second Syrian air force raid in a rebel-held part of Aleppo city killed at least 12 civilians on Saturday, the Observatory said.State news agency SANA did not report on these specific raids but, citing a military source, said the armed forces had targeted Islamic State in Aleppo province's eastern countryside and hit "terrorist organizations" on a road running north to the border, as well as to the south of Aleppo city and within the city itself.The military has been hit by a series of recent setbacks. Insurgents, including al-Qaida's Syrian wing Nusra Front, celebrated inside the last government-held town in Idlib province on Friday after capturing it the night before while Islamic State has secured its grip on the ancient central city of Palmyra.In Syria's northeast, clashes erupted between pro-government forces and Islamic State to the south and southeast of Hasaka city, the Observatory said on Friday, after the al-Qaida offshoot planted a bomb on the edge of the city. Hasaka province is strategically important as it lies next to Islamic State-held territory in Iraq, where the group is back on the offensive after losing the city of Tikrit at the start of the month.SANA said the Syrian military had foiled an attack on Hasaka and that it had been targeting Islamic State in the countryside with the help of Kurdish YPG militia. The YPG, which has fought the fiercest battles against Islamic State in the northeast, says it does not coordinate with the Syrian military.