The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, says Tuesday marked the "largest massacre in Syria" since April's chemical attack on Khan Sheikhoun in Idlib province when more than 80 people were killed.
In Thursday's deadliest incident, nine children and 12 adults were killed when a market in the town of Arbin was bombed, according to the Syrian Observatory. The previous night, at least 12 people were killed in Maarat a-Numan, al-Sheikho added, with bodies left "charred" by airstrikes there.
United Nations officials have called for an immediate cease-fire throughout Syria for at least one month in order to deliver humanitarian aid and services, as well as to evacuate the critically wounded.
Also, the letters noted that the Israeli airstrike coincided with a series of mortar strikes carried out by terrorists in Syria against civilian targets across the country.
A United States military official said the US-led coalition that still assists Kurdish-led forces in the hunt for surviving IS members in eastern Syria killed at least 100 pro-regime fighters overnight.
Russian Federation has waged a punishing aerial campaign against Syria's armed opposition since intervening in the civil war on the side of its ally, President Bashar Assad, in 2015.
The president of Syria Bashar-al-Assad in conjunction with the Russian Airforce and militias backed by Iran are pursuing military operations against the rebels in the last significant redoubts of the regions that have been in control of Assad's opponents in the western side of the country.
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Syrian government warplanes have ratcheted up their bombardment of Eastern Ghouta this week, leaving dozens dead and hundreds in need of medical care.
Separately, UN war crimes experts said they were investigating multiple reports of bombs allegedly containing chlorine gas being used against civilians in the rebel-held towns of Saraqeb in the northwestern province of Idlib and Douma in the Eastern Ghouta suburbs of Damascus.
More than 340,000 people have been killed and millions displaced since the conflict in Syria erupted in March 2011.
Activists say the campaign in Ghouta escalated after rebels there refused to join peace talks hosted by Russian Federation last month.
Thus far, the Damascus regime has gone unpunished militarily for its latest suspected chemical weapons attacks in January and February.
The U.S. State Department said it was "extremely concerned" about growing violence in Idlib and the eastern Ghouta area.
"That is not realistic", Russian Ambassador Vassily Nebenzia told reporters at the United Nations in NY.
"The general command of the armed forces holds Israel fully responsible for the unsafe consequences of its repeated, aggressive and uncalculated adventures", it added. The third one, attacked on Wednesday, is "said to have been producing chemical weapons".