School closures ordered by the Islamic State militant group have resulted in an estimated 670,000 children in Syria being deprived of an education, according to the United Nations. The U.N. children's agency said Tuesday that the group’s demand that schools stay closed until their curriculums could be changed was yet another reminder of the “terrible price” Syria’s children were paying amid the country’s chaos, Reuters reported.

The Islamic State, an al Qaeda offshoot that has seized territory in Syria and Iraq, began shutting schools in areas in eastern Syria in November. The closures were ordered pending a religious revision of the curriculum. "In December there was a decree of the Islamic State ordering the stoppage of education in areas under its control,"  UNICEF spokesman Christophe Boulierac said in a Tuesday news briefing in Geneva. "Islamic State said that the curriculum needed to be reshaped or re-conceived."

At least 160 children were killed and 343 injured in the 68 reported attacks on schools in Syria last year. These figures are likely an underestimate due to the challenges of obtaining data, Boulierac said. Between 1.3 million and 1.6 million Syrian children have been prevented from attending school as a result of the now five-year civil war, which has killed more than 200,000 and displaced half the country’s population, according to Agence France-Presse.

"In addition to lack of school access, attacks on schools, teachers and students are further horrific reminders of the terrible price Syria's children are paying in a crisis approaching its fifth year," Hanaa Singer, a UNICEF representative in Syria, said.

The militant group has already overhauled school curriculums in parts of Iraq. In September, students in Mosul returned to school to find that history, geography and literature lessons were eliminated, with a ban issued by the group only allowing religious studies to be taught, Bloomberg reported. A letter issued by the group also stated that the phrase “the Republic of Iraq or Syria wherever found or seen should be replaced with Islamic State” in textbooks. Such moves are seen as a reflection of the group’s efforts to entrench itself in the territories it has captured.