The army's bombing of a Syrian high school has shown that even attending the first day of school can be fatal for students, a rights watchdog said Tuesday.

On Sunday, the attack outside a school in the northern city of Raqa killed up to 16 people, mostly teachers and students.

Human Rights Watch researcher Priyanka Motaparthy said this was "the latest in a long string of government attacks that hit schools and killed students."

"These attacks have cost many children their lives, and have taught others that they risk death by going to school," she said in a statement.

"The blast wounds and flash burns visible on victims in videos and photographs, coupled with the body positions and few shrapnel wounds, indicates the use of fuel-air explosives (FAE), also known as 'vacuum bombs,'" HRW said.

The Syrian army has used this type of conventional weapon since 2012, it added.

The New York-based group said these bombs "should never be used in populated areas" because of their "highly indiscriminate" nature.

Attacks on schools "are serious violations of international humanitarian law (the laws of war), and individuals who intentionally commit such violations are responsible for war crimes."

The statement comes as a team of some 20 inspectors are set to arrive in Damascus to oversee the implementation of a UN Security Council resolution ordering the destruction of Syria's chemical arsenal.

"While the world tries to bring Syria's chemical weapons under control, government forces are killing civilians with other extremely powerful weapons," said Motaparthy.

More than 110,000 people have been killed in Syria's 30-month war, and millions more forced to flee their homes.

According to the UN, about half of the displaced are children, only a minority of whom have access to schooling this year.

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