UN envoy Kofi Annan meets Syria's Bashar al-Assad
UN-Arab League envoy Kofi Annan is holding talks with Syrian President Bashar al-Assad in Damascus.
Mr Annan's plan to end the country's conflict has been overshadowed by international revulsion at Friday's massacre in the Houla region.
Mr Annan called the massacre "an appalling moment with profound consequences".
Survivors have told the BBC of their shock and fear as regime forces entered their homes and killed their families.
Mr Annan said the Syrian government had to take "bold steps" to show it is serious about peace.
He said his "message of peace is not only for the government, but for everyone with a gun".
The BBC's Jim Muir in neighbouring Lebanon says it is make-or-break time for Mr Annan's peace plan, and he has to get something out of his visit to stop the drift towards a vicious sectarian civil war.
The former UN secretary general is expected to press President Assad to make good on earlier promises, and much will depend on the position taken by Syria's main international ally and diplomatic protector, Russia, our correspondent adds.
On Monday Mr Annan held talks with Foreign Minister Walid Muallem and the head of the UN observer mission in Syria, Major General Robert Mood.
Under Mr Annan's plan, both sides were to stop fighting on 12 April ahead of the deployment of monitors, and the government was to withdraw tanks and forces from civilian areas.
Russia, which has twice blocked UN Security Council resolutions backing action against the Damascus government, said on Monday that both sides bore responsibility for Friday's massacre.
"We are dealing with a situation in which both sides evidently had a hand in the deaths of innocent civilians," Russian Foreign Minister Sergei Lavrov said.
Western leaders have expressed horror at the killings, and the UK, France and US have all begun moves to raise diplomatic pressure on the Assad government.
France is convening another meeting of the so-called Friends of Syria group, which Russia does not take part in.
"The murderous folly of the Damascus regime represents a threat for regional security and its leaders will have to answer for their acts," said President Francois Hollande's office.
'I saw bodies'
Survivors who spoke to the BBC, and the local commander of the Free Syrian Army, said the people who carried out the killings were militiamen - shabbiha - from nearby Alawite villages.
Their accounts cannot be confirmed, but they are consistent with one another, and also with the reports given by activist groups on the ground in the immediate aftermath of the massacres, our correspondent says.
Syrian leaders insist that the massacre was the work of hundreds of armed rebels who massed in the area, and who carried out the killings to derail the peace process and provoke intervention by Western powers.
Several witnesses said they hid or played dead to survive.
UN observers who visited the village of Taldou where the massacre happened said they had found evidence of shelling from government forces.
They also confirmed that some of the 108 victims - many of whom were children - had been killed by close-range gunfire or knife attacks.
"We were in the house, they went in, the shabbiha and security, they went in with Kalashnikovs and automatic rifles," survivor Rasha Abdul Razaq told the BBC.
"They took us to a room and hit my father on the head with the back of a rifle and shot him straight in the chin."
Of 20 family members and friends in the house at the time, she said only four had survived.
Another resident, who asked to remain anonymous, said he hid in the attic as gunmen took his family outside and shot them.
"I opened the door, and I saw bodies, I couldn't recognise my kids from my brothers. It was indescribable. I have three children, I lost three children," he said.