At least 10 civilians, including five children, were killed in air strikes by Syrian government ally Russia on Monday in the northwestern Idlib province, the last major rebel-held territory.
This comes hours after Russia's Defence Ministry announced a unilateral ceasefire.
The Russian army launched air raids on a town in the province, largely controlled by former al-Qaeda affiliate Hay'et Tahrir al-Sham (HTS) group, claiming that the area was being used as a launchpad to fire rockets on Syrian airbase.
Fighting erupted in northwestern Syria last month breaking a truce brokered by Russia and Turkey in 2018.
At least 167 civilians have lost their lives in Idlib since April 25. The province is home to about three-million people.
The Syrian war has killed more than 370,00O people and displaced millions inside Syria and abroad since it began in March 2011.
The Syrian government has been accused of attacking civilians in Idlib. United Nations has also warned of a humanitarian catastrophe.
Syrian President Bashar al-Assad is trying to regain control over the last rebel-held province.
The multi-sided armed conflict in Syria has been raging since 2011, with Russia, Turkey and Iran being the guarantors of the ceasefire regime.
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