The US, UK and France have called on the UN to “urgently investigate” allegations from the opposition in Syria that the Assad regime has used chemical weapons in a mass attack that has left hundreds of civilians dead.

Washington yesterday said it was “deeply concerned” by the reports as the UN Security Council had an emergency meeting to discuss the claims of strikes on the opposition-held Damascus suburbs.

According to the UK-based Syrian Observatory for Human Rights, a pro-opposition monitoring group, the Assad regime unleashed a furious bombardment on rebel areas to the east and west of the capital from dawn yesterday. An activist had described the use of “toxic gas” in the eastern town of Zamalka, according to the Observatory, which said the death toll in the suburbs was at least 100. Other groups claimed as many as 1,000 died, although it was unclear how many were believed to be from poisoning.