UN observers have counted at least 90 bodies, including 32 children, after a Syrian government attack on a town.
UN mission head Maj-Gen Robert Mood told the BBC the killing in Houla was “indiscriminate and unforgivable”.
UK Foreign Secretary William Hague said he would seek a strong global response to the “appalling crime”. UN chief Ban Ki-moon said it was a “flagrant violation of international law”.
Syria’s government has blamed the deaths on “armed terrorist gangs”.
This is one of the bloodiest attacks in one area since the uprising against President Bashar al-Assad began in March 2011.