In Idlib, the last province under control of Syrian rebel forces, heavy fighting has been going on in recent days between opposition fighters and the Syrian government forces.
Rescuers and a monitoring group said warplanes bombed several towns in the Eastern Ghouta yesterday, causing buildings to collapse on residents.
It said Syrian government air strikes and artillery hit the towns of Douma, Beit Sawa and Hammouriyeh in the insurgent-controlled suburbs.
Earlier, Russian President Vladimir Putin and Turkish President Tayyip Erdogan agreed to strengthen coordination between the two countries' military and security services in Syria in the fight against terrorism, the Kremlin said in a statement.
Iran's President Hassan Rouhani has discussed the war in Syria with visiting Turkish Foreign Minister Mevlut Cavusoglu and offered Iranian assistance in helping allay Turkey's concerns about Syria's future.
On Monday, air strikes killed 30 people in Eastern Ghouta, the Observatory said.
From Damascus, the United Nations called for an immediate, month-long cease-fire in Syria in order to deliver critical humanitarian aid and medical care to civilians trapped by fighting in regions across the country.
Paulo Pinheiro, head of the International Commission of Inquiry on Syria, said the government siege of Eastern Ghouta featured "the international crimes of indiscriminate bombardment and deliberate starvation of the civilian population". The Syrian government denied deliberately attacking civilian targets, but provided no specifics on what they thought they were attacking.
He said: "There is a misperception that de-escalation areas have resulted in peace and stability". US -backed Kurdish-led forces have been fighting for control of Deir el-Zour with rival Russian-backed Syrian troops that are reinforced by Iranian-supported militias.
France's Foreign Ministry said it was concerned by the reports of chlorine used on civilians in Syria, but it was too soon to confirm them.
Turkey launched "Operation Olive Branch" two weeks ago targeting the YPG Kurdish militia in Afrin, opening a new front in Syria's seven-year-old, multi-sided civil war. Ankara fears the consolidation of Kurdish influence along its border and describes the Kurdish militia there as a terrorist group.
In a separate development more than 100 fighters aligned with Syrian President Bashar al-Assad were killed after USA -led coalition and coalition-backed local forces thwarted a large, apparently coordinated attack late on Wednesday and early on Thursday, a US official said. Turkey already has deployed troops in Idlib to such posts, and some of its soldiers have come under attack.