A chlorine gas attack was carried out on an enclave besieged by rebels on the outskirts of the Syrian capital, Damascus.
People in the eastern region of Ghouta, which is subject to daily bombing, reported a gas smell after a missile attack, the BBC learned.
Health workers said that six people were treated for minor respiratory problems.
About 400.000 people were besieged by government forces backed by Russia from the 2013.
There have been numerous reports of chlorine gas attacks since the civil war broke out in Syria, but the government has always denied the use of chemical agents.
The UN High Commissioner for Human Rights on January 10 declared that a wave of air strikes and ground attacks killed at least 85 civilians in the Eastern Ghouta from the December 31.
"In the Eastern Ghouta, where a paralyzing siege caused a humanitarian catastrophe, residential areas are being hit day and night by ground and air attacks, forcing civilians to hide in basements," Zeid Ra'ad al-Hussein said. in a statement.
The eastern Ghouta region is strategically important, as the rebels are able to launch missiles in the residential neighborhoods of Damascus. This also caused civilian deaths, according to state media.
Yusuf Ibrahim, a teacher who now lives underground in the rebel city of Harasta, just outside Eastern Ghouta, described the situation there.
"Today it is not as difficult as it was yesterday because there are no airplanes or raids until now, only heavy artillery bombardments ... like surface-to-surface rockets, which target Harasta's buildings and population," he said the BBC.
"The inhabitants of the city are all underground, they live in the basement or in the cellars because of such heavy bombardments.
"There are no means of life such as markets or good services".
Last week, humanitarian workers said that at least 10 hospitals in Syria areas held by rebels suffered direct air or artillery strikes over a period of 10 days.
A councilor of a charitable medical coalition told the BBC that the attacks were the most intense for a year.
The lack of food has led to many cases of severe malnutrition, and a UN report in November said that some residents in eastern Ghouta have been reduced to eating animal fodder and even garbage.
It is said that many have died of hunger.
The Eastern Ghouta is one of the many "desertification" areas in Syria announced by Russia, Iran and Turkey in the 2017.
Despite this, the area remained under the bombardments. Allied troops and militias also closed many of the routes used to smuggle food and medicine, leaving residents dependent on irregular aid deliveries.
