UN-Arab envoy Kofi Annan expressed "horror" at the Houla massacre of more than 100 people as he began a visit to Syria on Monday aimed at salvaging his battered peace plan.
Annan called the "tragic" massacre in the central town "an appalling moment with profound consequences."
The former UN chief said those responsible for the massacre must be held to account, and urged "everyone with a gun" to abide by his six-point blueprint to help end 15 months of bloodshed.
Syrian Foreign Minister Walid Muallem met Annan and the head of the UN observer mission in Syria, Major General Robert Mood.
Muallem explained "the truth of what is happening in Syria and the attacks against law and order which are aimed at sowing chaos... (despite) the reforms that Syria has adopted in all areas," the official SANA news agency reported.
World leaders have voiced outrage over the deaths of at least 108 people in the central town of Houla on Friday and Saturday, among them 49 children and 34 women, many blown to bits or shot dead at point blank range.
French President Francois Hollande's office said on Monday that Syria's leaders would have to answer for their "murderous folly."
His comments came after the UN Security Council -- where Syrian allies Russia and China wield veto powers -- on Sunday condemned the Damascus government's use of heavy artillery in the assault on Houla.
Annan told reporters in Damascus that he was "personally shocked and horrified by the tragic incident in Houla," saying the Security Council was right to condemn it.
He urged Damascus to take "bold steps" to signal it is serious in its intention to resolve the crisis peacefully.
"And this message of peace is not only for the government, but for everyone with a gun.
"The six-point plan has to be implemented comprehensively. And this is not happening. I intend to have serious and frank discussions with President Bashar al-Assad."
Those talks are scheduled for Tuesday, a Syrian official said on condition of anonymity.
Annan's peace plan was supposed to begin with a ceasefire from April 12, but this has been broken daily.
A watchdog said another 36 people were killed on Monday, a day after 87 died in one of the deadliest days of the putative truce.
The Syrian Observatory for Human Rights said 34 of Sunday's dead were killed in random shelling of the central city of Hama by troops retaliating for losses.
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It said clashes raged on Monday in regions including Idlib in the northwest and Daraa in the south, where an anti-regime revolt first erupted in March 2011. The Observatory says more than 13,000 people have been killed in violence since.
The UN Security Council's condemnation of the Syrian government's role in the Houla massacre did little to bring the international powers together to end the crisis.
Britain and France had proposed a text making an even stronger condemnation of the Assad government, but Russia would not agree on the wording and demanded a special meeting before approving the eventual text.
France said on Monday it would host a Friends of Syria meeting in Paris, after Hollande and British Prime Minister David Cameron held talks on the crisis, and slammed the Assad regime for its part in the Houla massacre.
"The murderous folly of the Damascus regime represents a threat for regional security and its leaders will have to answer for their acts," Hollande's office said.
And Britain summoned Syria's top diplomat in London to protest against the "sickening and evil" Houla massacre, the government said.
Saudi Arabia, which has advocated arming the rebels and called for Assad to quit, also condemned the massacre and urged the international community "to stop the bloodshed and the use of force against unarmed civilians," the official SPA news agency reported.
But Syria's UN envoy Bashar Jaafari said accusations of government responsibility were part of a "tsunami of lies."
Russia defended its key Middle East ally at the Security Council, and on Monday said both sides bore responsibility.
"Here we have a situation where both sides clearly had a hand in the fact that peaceful citizens were killed," said Foreign Minister Sergei Lavrov, adding "who is in power" in Syria was less important than "ending the violence."
Syria's opposition renewed its call to the international community to help Syrians defend themselves, a day after calling for a "battle of liberation" against the regime until the UN authorises foreign military intervention.
"The Syrian National Council calls (on) brothers and friends of the Syrian people to help before it's too late," the exiled group said in a statement.
The Free Syrian Army has warned that unless the international community took concrete action it would no longer be bound by Annan's plan.
Syria's Muslim Brotherhood lashed out at the Security Council for sending what it said was the "wrong message" to Damascus by only condemning the Houla massacre.
Human Rights Watch demanded that Annan push Assad's government to allow the UN-appointed Commission of Inquiry on Syria to investigate the massacre.
A growing concern for the international community is the more than 280 UN observers deployed in Syria as part of the peace plan.
UN Secretary General Ban Ki-moon said the Houla massacre had added to pressure on the monitors -- the first UN force to be thrown unarmed into a conflict with a non-existent ceasefire -- and that there was no "Plan B" if Annan's peace plan failed.