The main rebel group in Syria has said that it will not respect the cease-fire agreement if the UN doesn't act to protect civilians, according to news reports.
The Syrian government, meanwhile, has blamed the massacre in Houla on terrorists.
The rebel Free Syrian Army has warned that unless the UN Security Council takes concrete action to protect civilians from government violence, it will no longer be bound by the internationally brokered cease-fire agreement.
The rebel group's warning comes amid renewed violence on Sunday, with government forces reportedly targeting the opposition stronghold of Rastan with artillery and subjecting the central city of Hama to machinegun fire. Rebel forces retreated to Rastan earlier this year to escape a government military offensive against the city of Homs.
A bomb blast also hit a government security vehicle in the upscale Damascus suburb of Mazzeh on Sunday, according to the London-based Syrian Observatory for Human Rights and the activist Local Coordination Committees (LCC). The two groups reported that several security personnel were injured in the blast without giving further details.
Over the weekend, the UN called for the perpetrators of a massacre in the city of Houla to be held to account. The Gulf nation of Kuwait, meanwhile, began organizing an Arab League meeting to "take steps to put an end to the oppressive practices against the Syrian people."
Some 92 people were killed in the massacre in Houla on Friday, 32 of them children. The Free Syrian Army issued a statement on Saturday saying that killings were "taking place under the eyes of the UN observers," urging the international community to "announce the failure of the Annan plan."
"Annan's plan is going to hell," the group said in its release. Arab League-UN special envoy Kofi Annan brokered the agreement, which went into effect on April 12. It has been violated on a regular basis by rebel groups and the Syrian government since then.
The Syrian government on Sunday denied responsibility for the massacre in Houla, blaming terrorist groups for the violence.
"Terrorist groups from al Qaeda committed two heinous massacres against families in the countryside of Homs," the state news agency SANA said, quoting an unnamed official in the area.
Syrian Foreign Ministry Spokesman Jihad al-Makdissi, meanwhile, said that targeting defenseless civilians "was not the hallmark of the heroic Syrian army," promising a full investigation.
"We completely deny responsibility for this terrorist massacre against our people," Makdissi told a news conference.
"We have set up a military and legal committee to investigate," he said. "The results will emerge within days."