An alleged chemical weapons attack has killed dozens of people in eastern Syria, according to reports from aid workers in the area.
It came amid renewed shelling and fighting in the area.
Rescue and medical teams in Douma - the last rebel-held city in Eastern Ghouta - posted apparent photos of the victims, some of whom were shown foaming from the mouth.
The White Helmets group claimed civilians had been targeted by 'poison gas'.
The United Nations on Sunday said it was 'not in a position' to verify the reports, but said "any use of chemical weapons, if confirmed, is abhorrent".
Cases of suffocation between the civilians in residential neighborhood in the city #Douma after it was targeted by poison gas “Chlorine”. @SyriaCivilDefe teams are working to inspect the place and transfer the injured to medical centers. pic.twitter.com/07FrrKns31
— The White Helmets (@SyriaCivilDef) April 7, 2018
The Union of Medical Care and Relief Organisations (UOSSM) said victims had "experienced symptoms consistent with inhaling toxic gas".
The group claimed at least 70 people were dead.
A spokesperson added: "The death toll is expected to rise to well over 100 victims as Syrian Civil Defense teams have experienced extreme difficulty reaching victims due to the continued bombardment on Douma."
A joint statement from the Syrian American Medical Society and Syrian Civil Defence said at least 42 people were found dead in their homes.
They also reported that more than 500 people had been brought to hospital for treatment.
The statement adds: "Civil defense volunteers were unable to evacuate the bodies due to the intensity of the odor and the lack of protective equipment.
"The reported symptoms indicate that the victims suffocated from the exposure to toxic chemicals, most likely an organophosphate element."
Many of those reported to have been killed or injured were women and children.
Syrian state news agency SANA reported that the government denied any responsibility, with sources claiming the reports were 'fabrications'.
Smoke rises after the Syrian army's shelling targeted the Douma district in Eastern Ghouta countryside of Damascus, Syria. Picture by: Ammar Safarjalani/Xinhua News Agency/PA Images
UN Secretary-General Antonio Guterres is said to be "alarmed" by the allegations of chemical weapons use against civilians.
A spokesperson for the UN chief said: "While the United Nations is not in a position to verify these reports, the Secretary-General notes that any use of chemical weapons, if confirmed, is abhorrent, and requires a thorough investigation."
Amid strong statements from the US and UK in response to the reports from Syria, Russia claimed the allegations were 'fabricated'.
In comments quoted by the TASS news agency, a defence official said: "We strongly reject this information and confirm readiness after Douma is liberated from militants to send Russian specialists in radiation, chemical and biological protection to collect data to confirm that these statements are fabricated."
US President Donald Trump hit out at Russia and Iran following the attack:
Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price...
— Donald J. Trump (@realDonaldTrump) April 8, 2018
In a second tweet, he added: "Big price to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"
The US State Department had earlier strongly condemned the alleged attack.
In a statement, the department said: "Reports from a number of contacts and medical personnel on the ground indicate a potentially high number of casualties, including among families hiding in shelters. These reports, if confirmed, are horrifying and demand an immediate response by the international community.
"The Assad regime and its backers must be held accountable and any further attacks prevented immediately. Russia, with its unwavering support for the regime, ultimately bears responsibility for these brutal attacks, targeting of countless civilians, and the suffocation of Syria’s most vulnerable communities with chemical weapons."
It adds: "The United States calls on Russia to end this unmitigated support immediately and work with the international community to prevent further, barbaric chemical weapons attacks."
In a statement, a spokesperson for the UK's Foreign Office also called for an urgent investigation, saying: "These are very concerning reports of a chemical weapons attack with significant number of casualties, which if correct, are further proof of Assad’s brutality against innocent civilians and his backers’ callous disregard for international norms."
Syrian government forces have recaptured most of Eastern Ghouta, leaving just Douma in the hands of insurgent group Jaish al Islam.
Additional reporting by IRN