Kofi Annan, the UN's envoy to Syria, has warned that the country has reached a "tipping point" after the government was accused of massacring of 108 people, including 49 children.
Annan, speaking after talks with President Bashar al-Assad, called for the government to show "maximum restraint" one year since its troops began slaughtering peaceful protesters and citizens.
"We are at a tipping point. The Syrian people do not want the future to be one of bloodshed and division. Yet the killings continue and the abuses are still with us today.
"I appealed to him for bold steps now – not tomorrow, now."
He also called on opposition forces to "cease acts of violence".
The comments came as the UN Security Council prepared to meet on Wednesday to discuss the growing crisis.
France's new president Francois Hollande has said military intervention could not be ruled out in the Middle Eastern country, adding it was wrong to allow Assad's government to "massacre its own people". But the US has emphasised it is focusing on the "diplomatic tack."
Pentagon spokesperson George Little said the US had not been asked to plan for military intervention but commented: "We in the Department of Defence have a responsibility to look at the full spectrum of options and to make them available if they're requested."
On Tuesday the US, Germany, Britain, Canada, France and Australia all expelled Syrian diplomats in a mark of growing international revulsion at the country.
But despite the growing diplomatic pressure, Reuters reported that the Security Council permanent members are still deeply split.
"There are no signs Russia and China are ready to support tougher steps at the UN despite what happened in Houla," a diplomat told the news service.
The Times on Wednesday carried an editorial urging the British government to make the "most stringent action" to cause "economic pain" to Assad.
"What kind of country would Britain be, and what kind of people would young Syrians take us for, if we allowed the slaughter to continue? President Assad should know the period of 'do nothing' is over.."
On Tuesday the UN said victims of a massacre in Houla, Syria, including children, were "summarily executed."
Rupert Colville, spokesman for the United Nations High Commissioner for Human Rights, told journalists ahead of a crunch meeting between the UN's envoy for Syria Kofi Annan and the country's president Bashar al-Assad that most victims were murdered in an "abominable event."
"At this point it looks like entire families were shot in their houses.
"It's believed that under 20 of the 108 killings can be attributed to artillery and tank fire," he said.
"Most of the rest of the victims in Taldo, one of the areas of Houla, were summarily executed in two separate incidents.
"What is very clear is that this was an absolutely abominable event that happened in Houla and at least a substantial part of it was summary executions of civilians including women and children."
Reports from the town indicate children's throats were slit during the mass slaughter. Survivors told the BBC on Monday they had to play dead to survive after soldiers entered their homes.
Rasha Abdul Razaq said out of the 20 family members in her house, only four survived: "We were in the house, they went in, the shabiha and security, they went in with Kalashnikovs and automatic rifles.
"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."
One woman told Human Rights Watch she was sitting with her grandchildren when she heard gunshots and saw men dressed in military uniform walk into the house.
"The children, all aged between 10 and 14, were crying. I went down on the floor and tried to crawl so I could see what was happening.
"As I approached the door, I heard several gunshots. I was so terrified I couldn’t stand on my legs. I heard the soldiers leaving. I looked outside the room and saw all of my family members shot.
"They were shot in their bodies and their head. I was terrified to approach to see if they were alive. I kept crawling until I reached the back door. I went outside, and I ran away. I was in shock so I don’t know what happened later."
Syria's leaders had claimed its troops came under attack from heavily-armed rebels and were not to blame for the massacre in the town of Houla. However after an emergency meeting on Sunday night, the UN Security Council said the Syrian government was responsible for artillery and tank shelling of residential areas.
It also condemned "the killing of civilians by shooting at close range and by severe physical abuse", adding that the "outrageous use of force" violated international law.
Warning: some of these pictures are graphic