UN-Arab League envoy to Syria Kofi Annan has said the country has reached a "tipping point" after more than a year of conflict.
Mr Annan made the remarks after talks with Syrian President Bashar al-Assad.
Mr Annan told reporters the six-point international peace plan for Syria was not being implemented "as it must be".
Meanwhile, several Western states announced they were expelling Syrian diplomats following Friday's massacre in the Houla region, in which 108 died.
"I appealed to him [Mr Assad] for bold steps now - not tomorrow, now - to create momentum for the implementation of the plan," Mr Annan said.
Calling on the Syrian government and all government-backed militias to show "maximum restraint", he also asked the "armed opposition to cease acts of violence".
President Assad said the success of Mr Annan's peace plan depended on halting what he called terrorist actions and stopping arms-smuggling.
The United States, France, the UK, Germany, Italy, Spain, Canada, Australia, the Netherlands and Switzerland all took action on Monday to expel Syrian diplomats over Friday's killings in the Houla region.
Earlier, UN human rights spokesman Rupert Colville said initial investigations had suggested that most of those killed in the village of Taldou were summarily executed.
He said 49 children and 34 women were among the victims. UN observers who visited Taldou said many of the victims had been killed by close-range gunfire or knife attacks.
Eyewitnesses told the BBC that pro-government shabiha militiamen had carried out the killings. Survivors said they had hidden or played dead.
Syrian leaders insist that the massacre was the work of "terrorists", aiming to derail the peace process and provoke intervention by Western powers.
Violence continued on Tuesday, with nearly 50 people killed in various incidents, according to activists.
US state department spokesperson Victoria Nuland announced that the Syrian charge d'affaires in Washington had been given 72 hours to leave the country.
Speaking later, White House spokesman Jay Carney said the Obama administration remained opposed to military action in Syria, on the grounds that it might only lead to more carnage.
However, French President Francois Hollande, speaking in Paris, said an "armed intervention" was not being ruled out, provided it followed deliberation by the UN Security Council.
He said he would discuss Syria with his Russian counterpart, Vladimir Putin, when he visits Paris on Friday.
A meeting of the so-called Friends of Syria group is to meet in France in July, Mr Hollande's office said.
In other reaction
- Canada denounced the Syrian government's "heinous and murderous acts"
- Australia described the Houla massacre as a "hideous and brutal crime"
- Spain talked of "unacceptable repression"
- The Dutch declared Syria's ambassador to the Netherlands, who is also ambassador to Belgium and lives in Brussels, "persona non grata"
- Syria's ambassador to Switzerland was declared "persona non grata"
- Bulgaria said it was expelling Syria's interim ambassador and two other diplomats
Syria's charge d'affaires in London has been given seven days to leave.
News of the expulsions was welcomed by Ronya Kaysar, a representative of Syrian opposition groups based in Qatar.
It would put "great pressure on the Syrian regime", she told BBC News.
Despite the international show of protest, it is unclear whether the mass diplomatic expulsions will change much on the ground, Bridget Kendall, the BBC's diplomatic correspondent, says.
Russia, which supplies arms to the Syrian government and has blocked UN resolutions calling for action against Damascus, has blamed both sides for Friday's massacre.
Its Foreign Minister, Sergei Lavrov, expressed concern that "certain countries" were beginning to use the Houla massacre "as a pretext for voicing demands relating to the need for military measures to be taken".
Speaking to Mr Annan by telephone on Tuesday, he again urged an end to violence on all sides, and called for an impartial investigation into the killings in Houla under the aegis of the UN observer mission.