The Observatory said at least 22 government troops and allied militiamen and 40 ISIS fighters had been killed in clashes in the area in the past 24 hours.
The surprise offensive targeted pro-government troops at positions on the main road linking the capital of Damascus to Aleppo, UK-based war monitor The Syrian Observatory for Human Rights said on Thursday.
Syria's civil war began in 2011 after mass protests against the rule of President Bashar al-Assad and has killed hundreds of thousands of people, driven half the country's population from their homes and dragged in world powers. Most of the civilians had been beheaded.
Isis kills 50 in attack on two Syrian villages Beirut Islamic State has killed more than 50 people in an attack on two government-held villages in Hama province, central Syria.
State news agency Sana also reported the attack in Aqareb, saying 20 civilians were killed there, but made no mention of al-Mabujeh.
Rami Razzouk, a coroner at the hospital who inspected the bodies, said the children brought in were mostly dismembered, while most of the men died from shelling or heavy machine gun fire. Media reports and doctors in the area said some of the killed, which included women and children, were beheaded and others dismembered.
"Dozens of people are missing but it is not clear if they were kidnapped" by IS, the Observatory's chief Rami Abdurrahman said.
He said at least nine children were beaten with heavy objects such as bricks or stones on their heads or necks.
Isis, meanwhile, claimed via its news channel Amaq it had killed 100 government troops and captured two villages in Thursday's violence.
"It would be useful if this person was replaced", Cavusoglu said.
Separately, Syrian state television said Islamic State shelled a government-held district in the eastern province of Deir al-Zor also killed 13 people on Thursday, many of them children.
The IS-linked Aamaq news agency confirmed an attack is ongoing in the area without giving details. It identified residents as members of Assad's Alawite sect, an off-shoot of Shiite Islam. The Sunni extremists view Shiites as apostates deserving of death.
The Observatory said IS had seized control of Aqareb and part of Al-Mabujeh. A separate source in the hospital said 120 people were wounded.