Airstrikes killed at least 18 people, including seven children, in rebel-held Idlib region in northwestern Syria on Sunday, the Syrian Observatory for Human Rights and Reuters reported.

The northwest is the last major foothold of the rebellion against President Bashar al-Assad, who has vowed to recover the whole country but has made little or no gains in more than two months of military operations in the area.

State media also said that a train carrying phosphate in central Syria was derailed by a bomb blast east of Palmyra, causing injuries among the crew. It did not identify the perpetrators. Daesh is still active in that area.

In the northwest, the Observatory said Syrian government airstrikes killed 12 people, including five children, in the village of Urum al-Jawz in western Idlib province. Four more people, including two children, were killed in strikes on Kfarouma in the south of the province.

A Russian airstrike killed a civil defence volunteer in the town of Khan Sheikhoun, the Observatory said.

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The civil defence, also known as the White Helmets, identified him as Anas al-Diab, a volunteer and media activist it said was killed while documenting the airstrikes.

The Observatory said the deaths increased the number of civilians killed by the Syrian government or Russian bombardments in the northwest to 682 since late April. It said 53 civilians had been killed by rebel attacks on state-held areas in the same period.

Some 1,500 combatants on both sides have been killed in the same period, it said.

Syrian state news agency SANA said the Syrian army had repelled an attack by militant groups that had targeted army positions protecting the village of al-Qasabiya in southern Idlib.

SANA said army units destroyed vehicles belonging to the Nusra Front and its affiliates had been destroyed in the attack, during which the militants used suicide bombers.

The Nusra Front is today known as Tahrir al-Sham, a jihadist organisation that holds sway in the Idlib area.