Special envoy Kofi Annan is to visit Damascus on Monday, the day after the UN condemned Syria for its use of heavy weaponry in the town of Houla, where at least 108 people were killed.
Forty-nine children and 34 women were among Friday's dead, the UN said.
Syria's ambassador to the UN rejected what he called a "tsunami of lies" from some Security Council members.
Meanwhile, UK Foreign Secretary William Hague is going to Russia, which views Syria as a vital ally in the region.
The BBC's Daniel Sandford, in Moscow, says Mr Hague will argue that this could be the last opportunity to secure a "political transition" in Syria and avoid all-out civil war.
Russia and China have blocked previous attempts to impose UN sanctions on Syria.
The Syrian ambassador to London has also been summoned to the Foreign Office to hear the British government's condemnation of the massacre in Houla.
Fighting in Syria has continued despite the deployment of some 280 UN observers monitoring a ceasefire brokered by Mr Annan.
Opposition activists said at least 30 people were killed on Sunday when the army shelled the central city of Hama. These reports cannot be independently verified, as Syria severely restricts journalists' freedom of movement.
Sunday saw thousands of activists take to the streets in protest at events in Houla.
'Severe physical abuse'
On Sunday, the Security Council unanimously adopted the non-binding statement, calling for the Syrian government to withdraw its heavy weaponry from residential areas and return them to barracks.
"The members of the Security Council condemned in the strongest possible terms the killings... in the village of (Houla), near Homs, in attacks that involved a series of government artillery and tank shellings on a residential neighbourhood," the statement said.
"The members of the Security Council also condemned the killing of civilians by shooting at close range and by severe physical abuse," the statement continued.
"Such outrageous use of force against civilian population constitutes a violation of applicable international law."
But Syria's UN ambassador, Bashar Jaafari, accused some members of the council were trying to mislead the world about Syria's role in the massacre.
"Neither [UN observer mission head Maj Gen Robert] Mood nor anybody else told the Security Council in the informal session that he would blame the Syrian government forces for what happened.
"It is really pitiful and regrettable that some members of the council came out just a few minutes after Gen Mood had finished his briefing to mislead you, to tell you lies about what happened," he said.
Mr Jaafari said there had been deaths, but said they had been due to close-range assasinations rather than a result of shelling. Syria has blamed the massacre on "terrorists".
According to activists, Syrian troops used artillery to pound homes in Houla while pro-regime militia stormed the area, killing people on the streets and in their homes.
On a video link from Damascus, Maj-Gen Mood told the Security Council that 108 people had been killed and 300 injured in Houla - up from a previous figure of at least 90 dead.
The UN on Sunday also revised the number of children who died to 49, and said the casualties included 34 women.
Maj-Gen Mood told the BBC that UN monitors were continuing their investigations in Houla.
On Sunday, Syria refused permission for Mr Annan's deputy to travel to Damascus with him, a senior Arab League official told AFP.
Meanwhile, Mr Hague is due to hold talks in Moscow in an attempt to convince the Russians to press the Syrians to abide by Mr Annan's peace plan.
Our correspondent says Mr Hague's visit was arranged weeks ago but the timing in fortunate as diplomats wrestle with the Syrian crisis after Friday's massacre.
The meeting also comes after reports that a ship carrying Russian arms for the Syrian government arrived in Syria on Saturday were described as "credible" by Western diplomats.
Under Mr Annan's plan, both sides were to stop fighting on 12 April ahead of the deployment of monitors, while the government was to withdraw tanks and forces from civilian areas.
The unrest in Syria has killed at least 10,000 people since protests against President Bashar al-Assad broke out in March 2011.