Boris Johnson has condemned the Syrian regime's alleged use of chemical weapons in air strikes on the besieged city of Aleppo.
The Foreign Secretary said reports of chlorine gas attacks on opposition-held areas, if proven, are "utterly abhorrent".
He spoke after a Syrian government airstrike on the city's eastern Zabadieh neighbourhood, in which at least four barrel bombs were dropped, one of which purportedly released the gas.
The attack came hours after the Russian military, which is backing the Bashar Assad regime, promised a daily three-hour ceasefire to allow humanitarian aid into besieged areas.
The Organisation for the Prohibition of Chemical Weapons said that reports of possible chemical weapons use in Syria "are of great concern".
Opposition fighters in Aleppo last week launched a counter-offensive which broke the siege on the southern front-line.
Mr Johnson said: "While the siege has been broken, the Assad regime and Russia continue to bombard parts of the city and I am particularly concerned by reports of chlorine gas attacks which if proven, would be utterly abhorrent.
"We unequivocally condemn the use of chemical weapons by anyone anywhere and will work with the UN and other partners to establish the facts and hold those responsible to account.
"The international community risks failing the Syrian people if we do not do more to avert further devastation and tragedy.
"We must secure sustained humanitarian access and an end to the indiscriminate regime and Russian attacks on civilian areas, including medical centres.
"Yesterday I spoke to Sergey Lavrov, the Russian foreign minister, and urged him to do this.
"We must also continue to make clear to the Assad regime that there is no victory in the brutal torture of its own people. Ultimately, the solution lies in an enduring political settlement based on transition away from the Assad regime. And the UK will continue to make every effort to support this."
The reports of chlorine gas being used in the attack were made by a first responder and could not be independently verified.
The UK-based Syrian Observatory for Human Rights said it had received reports of two people killed and several people suffering breathing difficulties after the attack.