Britain is to push for a "strong international response" to reports of a massacre of civilians by Syrian forces, Foreign Secretary William Hague has said.
Children are thought to be among the more than 90 people killed in the Houla area of central Syria in the past 24 hours.
Mr Hague said the UK will press for an urgent session of the United Nations Security Council to co-ordinate the response to the "appalling" crime.
"There are credible and horrific reports that a large number of civilians have been massacred at the hands of Syrian forces in the town of Houla, including children," he said. "Our urgent priority is to establish a full account of this appalling crime and to move swiftly to ensure that those responsible are identified and held to account. We are consulting urgently with our allies on a strong international response, including at the UN Security Council, the EU and UN human rights bodies.
"We will be calling for an urgent session of the UN Security Council in the coming days."
Mr Hague urged President Bashar Assad's regime to grant "full and immediate" access to Houla for UN monitors and stop all military options, as demanded by special envoy Kofi Annan.
The Syrian Observatory for Human Rights said at least 90 people had been killed.
A local activist, giving his name as Abu Yazan, said 12 people died in shelling and 106 were killed when pro-regime thugs known as shabiha stormed the area.
The death toll is one of the highest for any single event since the popular uprising against Bashar Assad began in March 2011. The UN says more than 9,000 people have been killed, most of them civilians.
The White House later said it was horrified by the brutal attack. National Security Council spokeswoman Erin Pelton said the attack serves as a "vile testament to an illegitimate regime" of Assad. The White House said the Syrian regime is responding to peaceful political protest with "unspeakable and inhuman brutality".