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Russian warplanes dropped bombs containing an incendiary substance called thermite in attacks on the towns of Saraqeb in Idlib and al-Latamenah in Hama over the weekend, the Syrian Observatory for Human Rights told Reuters.

A rescue worker with the Syrian Civil Defence, also known as the White Helmets, said he had not heard of the use of thermite, but said the Assad regime and ally warplanes often dropped phosphorus munitions.

Neither Russia nor the Syrian regime commented on the incidents.

The Syrian Observatory for Human Rights said the Syrian government first used thermite in the Syrian conflict in June 2016. 

Incendiary munitions are designed to be hard to extinguish and contain a highly explosive blast which has a devastating effect in civilian areas.

On Sunday, videos emerged on social media from Saraqeb showing highly flammable materials hitting the ground.

The incendiary bombings came days after Washington carried out its first military action against Bashar al-Assad's government since the start of Syria's six-year war.

US warships in the Mediterranean launched a barrage of 59 cruise missiles at Shayrat airbase in Syria early on Friday, after 87 people including 31 children were killed in a suspected Syrian regime-sanctioned chemical attack on rebel-held Khan Sheikhun.

Following the US attack, allies of the Assad regime threatened reprisals against any party that carries out "aggression" against Syria. 

Syria's war has killed more than 320,000 people since it started in 2011 with the brutal repression of anti-government protests.

International rights organisations say the brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians amount to war crimes.