Worldwide concern has been growing over the fate of eastern Ghouta, where residents say they have been running out of food and medicine.
While UN officials say hundreds of thousands of people have been killed in the conflict, Syrian regime officials say the death toll is closer to 10,000.
State media said rebel fighters shelling the government-held capital killed a woman.
United Nations officials in Syria called for fighting to stop to enable aid deliveries and the evacuation of sick and wounded, listing seven areas of concern including northern Syria's Kurdish-led Afrin region, being targeted by a Turkish offensive.
Separately, UN war crimes experts said they were investigating several 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.
The UN has a plan to reach 700,000 people with relief in the next two months, if it can obtain permission to proceed. Syrian government forces have been on the offensive in Idlib in recent weeks but the push intensified after militants shot down a Russian Su-25 near the town of Saraqeb over the weekend. The military said the soldier was killed late Monday by a mortar-and-rocket attack that also wounded five Turkish soldiers and a civilian military contractor.More news: Indians will stop using 'Chief Wahoo' logo in 2019
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Mounting attacks in Idlib come at a time when the escalation of violence in eastern Ghouta has also magnified the longstanding humanitarian crisis in that besieged pocket on the north-eastern outskirts of Damascus.
In Idlib, another one of the remaning opposition-held areas under relentless bombardment, at least three more civilians were killed by airstrikes, according to sources from the White Helmets.
Rescue workers said at least nine people had suffered breathing problems from chemicals dropped from the air.
Iran urged Turkey on Monday to halt its two-week-old military offensive against the Kurdish YPG militia in northern Syria's Afrin region, which is adjacent to Idlib.
Defying the USA warnings, Turkish President Recep Tayyip Erdogan on Tuesday vowed that Turkish troops would expand cross-border military operations from Syria's Afrin to Manbij. An offensive by Ankara to rout USA -backed Syrian Kurdish fighters from the northern enclave of Afrin is now in its third week.
The regime move came after one Turkish soldier was killed at an observation point in Idlib, where Russian Federation and Turkey cooperate to establish a de-escalation zone as part of the Astana agreement.
"We implore the global community to speak with one voice, taking every opportunity to publicly pressure the Assad regime, and its supporters, to cease its use of chemical weapons", spokeswoman Heather Nauert said in a statement Monday night.