The Syrian government has denied responsibility for an attack that killed at least 85 people, including 34 children, saying that it was carried out by "terrorists".
Jihad Makdissi, the spokesman for the Syrian foreign ministry, also lashed out at foreign leaders for accusing the government of having committed atrocities "without any evidence".
He told a press conference that "hundreds of armed men" had attacked Houla, a cluster of villages in Homs province, on Friday and clashed with government forces.
"They used heavy weapons and anti-tank rockets," he said on Sunday. "That's why we lost some of our men from the security forces."
Makdissi said Syrian forces were trying to defend themselves but insisted that "no Syrian artillery or heavy weaponry" had been used in Houla.
"Syria will continue defending its citizens," he said.
Activists in the region said Houla was hit by artillery barrage and that pro-government thugs, known as shabiha, stabbed women and children in their houses.
Syrian forces shot and killed at least two men on Sunday as protests broke out to condemn the Houla massacre, opposition activists said.
The two were killed in the Damascus suburbs of Yalda and Daraya, home to thousands of refugees who have fled a military crackdown on the central province of Homs.
Major General Robert Mood, the chief of the UN observer mission deployed to Syria, said monitors touring the area had counted 85 bodies, including 34 children under the age of 10 and seven women.
"Whoever started, whoever responded and whoever supported this deplorable act of violence should be held responsible." Mood said about Friday's assault.
He said that a residential area had been hit with a range of weapons, including "rifles, machine guns, artillery shells, tank shells," but stressed that the circumstances that "led to the tragic deaths" were still unclear.
"Whatever I learned on the ground in Syria ... is that I should not jump to conclusions."
Earlier, UN chief Ban Ki-moon and UN-Arab League peace envoy Kofi Annan condemned the assault, calling it an "appalling and brutal" breach of international law.
"This appalling and brutal crime, involving indiscriminate and disproportionate use of force, is a flagrant violation of international law and of the commitments of the Syrian government to cease the use of heavy weapons in population centres and violence in all its forms," a statement issued on behalf of the two officials said.
Houla has been the scene of frequent anti-government protests since the uprising against Syrian President Bashar al-Assad began in March last year. The town has also become a hub for opposition fighters.
Hadi al-Abdallah, speaking from Homs, said Houla was under the control of the Free Syrian Army, which meant government troops could not enter the town. Instead, they were launching shells from a distance in a bid to defeat the rebels.
Arab League foreign ministers are to hold an emergency meeting in the wake of the killings, the bloc's current president Kuwait said on Sunday.
"Kuwait will contact members of the Arab League to hold an emergency ministerial meeting to study the situation and take measures to put an end to the oppressive practices against the Syrian people," a foreign ministry statement cited by the official KUNA news agency said.
Hillary Clinton, the US secretary of state, condemned the killings "in the strongest possible terms" and said in a statement that "the United States will work with the international community to intensify our pressure on Assad and his cronies, whose rule by murder and fear must come to an end".
William Hague, the British foreign secretary, said "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".
London would seek an urgent session of the Security Council in coming days, in response to "credible and horrific reports that a large number of civilians have been massacred" by Syrian forces in Houla.
A peace plan drawn up by Annan, which took effect on April 12, stipulated that UN observers be deployed in the country to monitor a ceasefire. But both government forces and opposition fighters have continued attacks.
More than 13,000 people have been killed in the country since the uprising began, Rami Abdel Rahman of the Syrian Observatory for Human Rights told AFP news agency on Sunday.
"In total, 13,004 people were killed," Abdel Rahman said, adding that 9,183 of them were civilians. Another 3,072 were regime troops, and 749 were army defectors, he added.
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