U.S. Defense Secretary Jim Mattis said last week that the Syrian government had repeatedly used chlorine as a weapon, and Washington was also concerned about the potential use of sarin gas.
Russia, Assad's main backer, retorted that "no perpetrators have been identified" and accused the United States of orchestrating a "propaganda campaign" against the Syrian government.
Almost 200 people were wounded, and rescuers said many of those probably will not survive because of lack of medical supplies.
"Meanwhile, fighting and retaliatory shelling from all parties are impacting civilians in this region and Damascus, causing scores of deaths and injuries", said their statement, released before the latest casualty tolls emerged on Tuesday. "We don't know what to say".
The Britain-based Syrian Observatory for Human Rights said 10 towns in the Eastern Ghouta region outside Damascus were bombed on Tuesday. Al-Qaida-linked militants said they downed the Russian Su-25 in Idlib on Saturday by a portable air defense missile.
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Activists say the campaign in Ghouta escalated after rebels there refused to join peace talks hosted by Russian Federation last month.
In a statement, Paulo Pinheiro said the spiraling violence had made "a mockery of the so-called "de-escalation zones" - an agreement reached a year ago among Russia, Iran and Turkey to stabilize the lines of conflict and open corridors for humanitarian aid. Though Assad appears to have survived the insurgency, his forces continue to bomb and besiege opposition pockets around the country, including in Eastern Ghouta and Idlib.
Attacks on Turkish military forces in the northwest of Syria has left one soldier killed and five others injured on Monday.
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.
The other Turkish soldier was killed at an observation point in the rebel-held Idlib, where Russian Federation and Turkey are in works to establish a de-escalation zone as part of the Astana agreement.