Air strikes on the largely opposition and rebel-controlled region of northern Idlib followed air raids on Tuesday night and Wednesday that killed 23 civilians.
The State Department said it was assessing indications that the regime used chemical weapons on Sunday during its offensive in Idlib, the most significant remaining holdout of rebels.
Trapped civilians fear the Syrian regime is destroying vital facilities, including hospitals and rescue organisations, to clear the way for brutal ground assaults.
Dozens have died since April 26 in such attacks, war monitor says. Schools, health facilities and residential areas have been hit by the "worst barrel bombing campaign" in opposition-held areas in northwestern Syria in 15 months, UN says.
Syrian state media say militants have attacked a military post belonging to the regime forces in the country's northwest, setting off clashes that killed three people.
Syrian regime shelling has killed at least nine people in Idlib, according to a monitor.
Regime media says militant shells have wounded dozens in Aleppo.
Attacks were intensified on villages and towns in northern Hama, southern Idlib and Latakia — areas that fall within a demilitarised zone agreed on last September between Russia and Turkey.
In a Wall Street Journal article, Turkish President Recep Tayyip Erdogan called on the international community to take action, and warned that "the entire world stands to pay the price" otherwise.
Two people died after the attacks in Syria's Hama province at the end of March 2017. Dozens of people suffered severe respiratory symptoms.
A report by independent United Nations monitors claims that North Korea had an military cooperation with Syria and Myanmar.
SOHR says at least 34 people killed and tens more injured in an explosion in the city of Idlib. Earlier, regime forces recaptured a strategically important town in the province of Idlib in the latest advance into opposition-held territory.
Offensive against rebels' largest remaining stronghold has advanced in Syria. The UN OCHA says more than 60,000 people have been forced to leave their homes since early November.
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