At least 57 people were killed in airstrikes allegedly carried out by the US-led coalition against a prison run by the Islamic State terror organization in the Syrian city of al-Mayadin, a British-based war monitor said.The Syrian Observatory for Human Rights (SOHR) said on Tuesday that the air raids killed 42 civilians and 15 guards and Islamic State prisoners on Monday, Efe news reported.The death toll could increase as there are people still missing under the rubble, SOHR said, adding that the prison had about 100 detainees, including civilians and IS militants.Before turning it into a prison, the building was the home of the leader of al-Qaida affiliate in Syria who was assassinated by Islamic State. The facilities were divided into a special section for civilians and another for IS members.The international coalition began its military campaign in Syria on September 23, 2014, almost three months after the extremists proclaimed a caliphate in Syria and Iraq.Along with the Syrian Democratic Forces, led by Kurdish militias, the coalition is currently taking part in an offensive against IS positions in al-Raqqa province, the terror organization's main stronghold in Syria.