UK Foreign Secretary William Hague has called for an emergency meeting of the UN Security Council this week.
The UN has confirmed the deaths of at least 90 people in Houla, including 32 children under the age of 10.
Syria's foreign ministry denied army involvement, blaming "terrorists".
Spokesman Jihad Makdissi said there was a clash in the area on Friday afternoon between security forces and armed terrorists.
He said hundreds of armed men then gathered, armed with heavy weapons including mortars and anti-tank weapons, and attacked Houla.
The government had sent a team to investigate the killings, he added.
However, activists say the Syrian military bombarded Houla after demonstrations.
Activists say some of the victims died by shelling, while others were summarily executed by the regime militia known as the "shabiha".