Damascus - Islamic State group jihadists killed nine Syrian students when they shelled a school in the eastern city of Deir Ezzor on Tuesday, state news agency SANA reported.
The Syrian Observatory for Human Rights said nine girls were killed and another 20 wounded, mostly students from the regime-held district of Hrabesh.
"The toll is likely to worsen as some of the injured are in serious condition," the Observatory's Rami Abdel Rahman told AFP.
The Syrian government condemned the attack.
"The terrorist rockets will not prevent us from continuing our mission of education," said Prime Minister Wael al-Halaqi.
The Islamic State group has controlled nearly all of oil-rich Deir Ezzor province since 2013, but half of the regional capital remains in the government's hands.
In recent weeks, the US-led coalition and Russia's military have targeted IS jihadists in the province with air strikes.
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In the west of the country, at least 20 air strikes likely to have been carried out by Russia caused injuries in Latakia province, said the Observatory.
Elsewhere in Latakia, forces loyal to President Bashar al-Assad fought fierce battles with Islamist rebels, leaving several dead on both sides, it added.
The Syrian government has been trying for months to recapture rebel-controlled areas of the coastal province.
Last week, troops and allied militia pushed rebel fighters from a hilltop, Jabal Nuba, which overlooks a strategic highway in Assad's heartland.
In the northern province of Aleppo, meanwhile, five people were killed and dozens wounded over 24 hours in air raids on the town of Al-Bab, said the Observatory.
The Britain-based monitor, which relies on a network of sources on the ground, also said IS carried out nine execution-style killings in the same province during the past few days.
Aleppo province is almost entirely in the hands of Al-Nusra Front, the Syrian offshoot of Al-Qaeda, and its Islamist allies, as well as IS.
The conflict in Syria has killed more than 250,000 people and forced millions to flee their homes since it broke out in March 2011.