Several girls at an elementary school were the latest victims of Islamic State terror in Syria Tuesday, when activists said militants shelled the school, killing at least nine students.

As many as 20 other students at the all-girls school were wounded, some of them seriously, according to the Syrian Observatory for Human Rights, which claimed ISIS was responsible.

Syrian TV blamed "terrorists" -- the term the government uses for all armed groups fighting against President Bashar Assad's forces.

The school is located in the northeastern city of Deir el-Zour, which ISIS and Syrian government forces have fought to control. Analysts say ISIS has controlled much of the surrounding oil-rich province since 2013, but the government has gained ground inside the city.

"The terrorist rockets will not prevent us from continuing our mission of education," SyrianPrime Minister Wael al-Halaqi said, according to AFP.

Activists said the attackers fired multiple mortar shells hitting several parts of the school.

The terror network has shown little mercy even for innocent children. Attacks by ISIS-affiliated Boko Haram in northeastern Nigeria and neighboring countries forced more than 1 million children out of school, heightening the risk they will be abused, abducted or recruited by armed groups, the United Nations children's agency said Tuesday.

The bloody fighting has forced more than 2,000 schools to close in Nigeria, Cameroon, Chad and Niger, the agency said. Some have been looted or set on fire.

"The conflict has been a huge blow for education in the region, and violence has kept many children out of the classroom for more than a year, putting them at risk of dropping out of school altogether," said Manuel Fontaine, UNICEF's West and Central Africa Regional Director. Before the crisis, an estimated 11 million children of primary school age were already out of school in the four neighboring countries where Boko Haram stages attacks, the agency said.

The Associated Press contributed to this report.