Several Syrian Christians have been killed in the town of al-Qaryatayn in central Syria by militants from the Islamic State group, BBC reported Sunday. Earlier this week, the town was retaken by Russian-backed Syrian forces and their allies.

The head of the Syrian Orthodox Church, Patriarch Ignatius Aphrem II, said that 21 Christians were killed by the militant organization, also known as ISIS, after it captured the city last August, BBC reported. About 300 Christians were living in the city when the killings took place.

The patriarch added that those who were killed included three women while some died while trying to escape. Others were killed for breaking the rules of the “dhimmi contracts,” which needed them to submit themselves to Islam, the report said.

The church official said that five more people are believed to be missing or dead. The remainder of the group was reunited with their families after negotiations and ransom payments. The patriarch also said that the terrorist group had planned to sell the Christian girls to slavery, according to BBC.

When ISIS took over al-Qaryatayn, the extremist group abducted hundreds of people including several Christians. The town lies about 60 miles west of Palmyra, which was captured by ISIS last May in the group’s first major offensive. 

Syrian President Bashar Assad’s forces have made significant strides to recapture the area under ISIS control and since the ceasefire was implemented last month, fighting in the region has significantly reduced. A report on Sunday by Reuters said that the Russian air force and the Syrian army are also preparing for a joint operation that would free the northwestern city of Aleppo, the largest city in Syria and a commercial hub, different parts of which are controlled by the government forces and the rebels.

“We, together with our Russian partners, are preparing for an operation to liberate Aleppo and to block all illegal armed groups which have not joined or have broken the ceasefire deal,” Syrian Prime Minister Wael al-Halaki reportedly said Sunday.