Kofi Annan, the UN-Arab special envoy, has held talks in Damascus with Bashar al-Assad, Syria's president, amid international horror at a massacre of over 100 people.
Anna, a former UN secretary-general, met Assad in the Syrian capital on Tuesday, having arrived in the country a day earlier.
As he began a visit to Syria, Annan called the "tragic" massacre in the central town" of Houla an appalling moment with profound consequences".
He said those responsible 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.
Walid al-Muallem, Syria's foreign minister, met Annan and the head of the UN observer mission in Syria, Major-General Robert Mood, on Monday.
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 of whom had been shot dead at point blank range or died in explosions.
Activists said several children had also been stabbed to death.
The UN human rights office said on Tuesday that most of the people who were killed were summarily executed in two separate incidents.
Rupert Colville, spokesman for the UN High Commissioner for Human Rights, said that UN monitors found that less than 20 of the 108 people killed died from artillery fire. He said that most of the others were executed in two separate incidents.
In what could prove to be a signficant development, staunch ally Russia has also condemned the government for the attacks.
"The government bears the main responsibility for what is going on," Sergei Lavrov, the Russian foreign minister, said in Moscow following a meeting with William Hague, his British counterpart, on Monday. "Any government in any country bears responsibility for the security of its citizens."
French President Francois Hollande's office said on Monday that Syria's leaders would have to answer for their "murderous folly".
Pope Benedict XVI was "pained" by the massacre and called on religious communities in Syria to cooperate to bring peace to the violence-wracked country.
Canada's foreign minister called on the UN Security Council to take "stronger diplomatic action," including economic sanctions against the Syrian government over its "senseless slaughter of its own people".
The comments came after the 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 said 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 Syria 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".
Human Rights Watch, the US-based organisation, demanded that Annan push Assad's government to allow the UN-appointed Commission of Inquiry on Syria to investigate the massacre.
Annan's peace plan was supposed to begin with a ceasefire from April 12, but this has been broken daily.
On Tuesday, gunfire from a Syrian border post killed one Lebanese citizen and wounded at least three others, the Lebanese state news agency reported.
A Syrian rights watchdog group said another 64 people were killed throughout the country on Monday, a day after 87 died despite the putative truce.
In-depth coverage of escalating violence across Syria
The UK-based Syrian Observatory for Human Rights (SOHR) said 34 of Sunday's dead were killed in random shelling of the central city of Hama by troops retaliating for losses.
The SOHR says more than 13,000 people have been killed in violence since the outbreak of the revolt of the Assad regime in March last year.
The Security Council's condemnation of the Syrian government's role in the Houla massacre has done 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 David Cameron, UK prime minister, held talks on the crisis, and condemned the Assad government 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.