* Militants order school closures in three Syrian provinces
* Curriculum to conform with "religious rules" (Adds details)
GENEVA, Jan 6 (Reuters) - An estimated 670,000 children in Syria are being deprived of education after Islamic State forces ordered schools closed while the curriculum is made to conform with religious rules, the U.N. Children's Fund (UNICEF) said on Tuesday.
Islamic State, an offshoot of al Qaeda which has recruited foreign fighters, has seized land in Syria and Iraq, imposing its strict reading of Islamic law.
In November it shut schools in areas it controls in eastern Syria pending a religious revision of the curriculum. . The group stands accused of massacres, sexually enslaving women and girls and recruiting children as fighters.
"In December there was a decree of the Islamic State ordering the stoppage of education in areas under its control," UNICEF spokesman Christophe Boulierac told a news briefing.
The militant group decreed that schools be closed until the school curriculum had been made "compliant with the religious rules," he told Reuters.
Children enrolled in primary and secondary schools in Raqqa, and rural areas of Deir al-Zor and Aleppo provinces are affected by the closures, he said. Teachers must undergo retraining.
In all, 4.3 million Syrian children are enrolled nationwide this school year, according to the education ministry, but between 2.1 million and 2.4 million are currently either out of school or attending classes irregularly, UNICEF said.
UNICEF also said that at least 160 children were killed and 343 wounded in attacks on schools across Syria last year. The toll was probably an under-estimate due to difficulties of access and obtaining data, Boulierac said.
"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, UNICEF representative in Syria, said in a statement.
Islamic State has been the target of U.S.-led air strikes in both Syria and Iraq since September. (Reporting by Stephanie Nebehay; editing by John Stonestreet and Dominic Evans)