Syrian troops today shelled the central region of Houla where more than 100 people were massacred last week, activists said.
At least one person was killed in the latest violence and scores fled in fear of more government attacks.
The shelling came as UN Secretary-General Ban Ki-moon called on Syria to stop its attacks, saying UN observers monitoring the ceasefire were not there to watch the killings of innocent people.
Meanwhile, the Free Syrian Army group of army defectors warned the Syrian government that if it does not abide by the UN plan by ceasing fire and pulling out troops from residential areas by noon tomorrow, the group will defend the people.
The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees activist groups said government troops unleashed heavy machine guns but also used mortars in Houla, a collection of poor farming villages in the central Homs province. A young man was killed by sniper fire.
Survivors of the Houla massacre have blamed pro-regime gunmen for at least some of carnage that began on Friday and left 108 people dead, many of them children and women. The Syrian government denied its troops were behind the killings and blamed "armed terrorists."
The Houla massacre was one of the deadliest incidents since the uprising against President Bashar Assad's regime started in March last year. The UN says more than 9,000 people have been killed in the past 15 months while activists put the number at about 13,000.
The Observatory reported that Houla residents were fleeing today to nearby towns and villages "fearing a new massacre."
In the wake of last week's massacre, the United States, Western and Asian nations expelled Syrian diplomats in protest.
Mr Ban, speaking in Istanbul, was referring about the nearly 300 UN observers who were deployed around Syria to monitor a ceasefire that went into effect on April 12, as part of an international peace plan negotiated by international envoy Kofi Annan. Despite the ceasefire violence continued almost daily.
Although the Free Syria Army claims that it has so far been abiding by Annan's plan, Syrian Deputy Foreign Minister Faisal Mekdad said gunmen have violated the ceasefire more than 3,500 times.
Also today, Syria's state-run TV said that 500 people who had got involved in recent events in Syria have been released from detention.
In Damascus, the Syria International Islamic Bank, or SIIB, criticised the latest sanctions imposed yesterday by the Obama administration as "irrational and unjustified."
The Treasury Department said yesterday that SIIB has been acting as a front for other Syrian financial institutions seeking to circumvent sanctions. The new penalties will prohibit the bank from engaging in financial transactions in the US.
* Iranian President Mahmoud Ahmadinejad says that those responsible for the Houla massacre should be punished, in unusually harsh criticism from a staunch ally.
Mr Ahmadinejad declined to say in an interview with the France 24 television station who he believed was behind the attack, but added: "It seems unbelievable to me that a government would engage in killing its own people .... (but) I'm not excluding anyone from this responsibility," he said.
"We (in Iran) are quite disappointed about this," he said from Tehran through a translator. "Any individual who committed these murders is guilty ... The people responsible for this massacre must be punished, must be sanctioned."
Every day of slaughter in Syria is strengthening the case for tougher international action, said US Secretary of State Hillary Clinton said. But she stressed that military intervention would need international support, including from Syrian ally Russia.
Speaking to Danish university students, Mrs Clinton noted that UN and international backing made possible last year's international coalition against Muammar Gaddafi in Libya but that Russia and China are standing in the way of similar action in Syria. She also suggested that a military effort against Mr Assad's regime would be far more difficult.
Asked when the US might opt for a military option, Mrs Clinton said: "Every day that goes by makes the argument for it stronger," even if Moscow in particular remained unconvinced. She said Washington would continue pressing its case to Russia, which has promised to block any moves at the United Nations to obtain a military mandate for intervention.