British Foreign Secretary William Hague said he was "deeply concerned" by reports of a chemical weapons attack on rebel-held areas near Damascus.
"These reports are uncorroborated and we are urgently seeking more information," he said in a statement. "But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria."
The BBC reported "hundreds" of people were killed early Wednesday. It said it was not able to verify footage it's seen of the incident, but said it believed the accounts were genuine.
U.N. weapons inspectors arrived in Syria last week to investigate claims of chemical weapons use by both parties to the conflict. The spokesman for U.N. Secretary-General Ban Ki-moon used Twitter to state the world body was "trying to find out more" about the incident.
The French and Turkish governments were swift to react to the allegations. The Turkish Foreign Ministry said the attack would constitute a "crime against humanity" if verified.
The official Syrian Arab News Agency said the allegations were "untrue." Hague, however, said he wanted the Syrian government to let weapons inspectors examine the scene of the attack.
"Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account," he said. "The U.K. will be raising this incident at the U.N. Security Council."