Paulo Pinheiro, head of the U.N. Commission of Inquiry on Syria, said the commission had "alarmingly" received several reports that chlorine gas had been used in attacks on the towns of Saraqeb, in the northwestern Idlib Province, and Douma in Eastern Ghouta near Damascus.
Paulo Pinheiro, in a statement, added that the spiralling violence in Syria had made "a mockery of the so-called "de-escalation zones" - an agreement previous year between Russia, Iran, and Turkey to stabilise the lines of conflict and open corridors for urgently needed humanitarian relief.
Syrian regime airstrikes there killed at least 63 people and injured more than 190 others on Tuesday, the UK-based Syrian Observatory for Human Rights said.
"The foreign ministry condemns the false claims by the United States that the Syrian government used chemical weapons in Eastern Ghouta", a foreign ministry source said, cited by Syrian state news agency SANA.
Insurgent shelling of government-held Damascus killed three people, the Observatory and Syrian state media reported.
The United Nations in Syria on Tuesday called for a one-month ceasefire in the country to allow the delivery of desperately needed supplies and medical care to civilians caught up in fighting.
Syria has been locked in a devastating civil war since March 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests with unexpected ferocity.
The UN has a plan to reach 700,000 people with relief in the next two months, if it can obtain permission to proceed.
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 a broadcast with the word "urgent" superimposed, it said at least 30 people had been killed and that airstrikes are still underway in the Ghouta.
Rescue workers said at least nine people had suffered breathing problems from chemicals dropped from the air.
Turkey began deploying forces in an observer role to the rebel-held Idlib in October as part of a "de-escalation" agreement with Iran and Russian Federation to stabilize the lines of conflict in the war-torn country.
Meanwhile, Turkish operations against Syrian Kurdish forces and USA and Russian-backed operations against the Islamic State group have compounded the suffering of Syrians in the country's north and east.
Turkey's military says a Turkish soldier was killed in an attack in Syria's Idlib province where the Turkish troops are establishing an observation post. An offensive by Ankara to rout US -backed Syrian Kurdish fighters from the northern enclave of Afrin is now in its third week.
Idlib is one of the last main strongholds of anti-Assad rebels who have been driven from most of their bastions in Syria since Russian Federation joined the war on the side of the Damascus government in 2015.