At least nine schoolgirls were killed and 20 others wounded when mortar shells recently hit the Harabesh primary school for girls in the Syrian city of Deir el-Zour, the Syrian Observatory for Human Rights (OSDH) reported.

OSDH believes that Islamic State (ISIS) militants were responsible for the mortar attacks and the deaths of the children. Syrian TV also blamed "terrorists"—the term the government uses for all armed groups fighting against President Bashar Assad's forces—as those behind the attacks, according to Fox News.

"The toll is likely to worsen as some injured are in serious condition," OSDH Director Rami Abdel Rahman said, AFP reported.

According to activists, the attackers fired multiple mortar shells hitting several parts of the school.

The Syrian government immediately condemned the attack. "The terrorist rockets will not prevent us from continuing our mission of education," Syrian Prime Minister Wael al-Halaqi said, according to AFP.

ISIS has been in control of almost all of oil-rich Deir Ezzor province since 2013, but half of the regional capital remains in government hands.

State media reported that since the air campaign against the ISIS intensified, the terror group has frequently targeted the schools. Last week, intense air and missile strikes on a school district and other areas in insurgent-held Damascus suburbs killed dozens, the Daily Mail reported.

On the same day, state media said mortar attacks targeting residential neighbourhoods of Damascus had killed three and wounded at least 30 people, most of them students.

The attacks come just a day after reports emerged of rocket and mortar fire into several regime-controlled areas of Syria, killing six civilians.

Earlier attacks by ISIS-affiliated Boko Haram in northeastern Nigeria and neighbouring countries also 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 agency added that the bloody fighting has forced more than 2,000 schools to close in Nigeria, Cameroon, Chad and Niger. 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 neighbouring countries where Boko Haram stages attacks, the agency said.