Britain condemned an airstrike on a bakery in Syria which is believed to have killed more than 60 people queuing to buy bread.
Foreign Office minister Alistair Burt said that, if verified, the attack would be "the most recent in a long line of human rights violations and abuses" committed by the regime of President Bashar Assad.
Those responsible should be held to account, said Mr Burt, who said that the incident made clear "the urgent need for a political transition and end to the violence". The attack in the town of Halfaya overshadowed the visit to Damascus of international peace envoy Lakhdar Brahimi, who held talks with Assad.
There was little sign of progress in the efforts of the envoy for the United Nations and Arab League to broker an end to the conflict during the two-day visit.
Following the discussions at the Presidential palace, Mr Brahimi told reporters: "The situation in Syria is still worrying and we hope that all the parties will go toward the solution that the Syrian people are hoping for and look forward to."
Syria's state news agency quoted Assad as saying his Government supports "any effort in the interest of the Syrian people which preserves the homeland's sovereignty and independence."
Anti-regime activists reported that a government warplane targeted a bakery in the rebel-held town of Halfaya on Sunday.
Amateur videos posted online showed many dead and wounded bodies lying in a street.
The Syrian Observatory for Human Rights put the death toll from the incident at 60, and said it had collected the names of 40 men and three women killed in Halfaya and had seen photos of the dead bodies of a further 15 unidentified men.
In a statement released in London, Mr Burt said: "The attack highlights the urgent need for a political transition and end to the violence, and for those responsible for these appalling acts to be held to account."