The Syrian army shelled residential areas and unleashed gunmen in attacks which killed at least 18 people, including an eight-year-old boy, a human rights group said.
The shelling of neighbourhoods evoked memories of the Assad regime's brutal, 40-year legacy of crushing dissent.
Syrian activists and protesters involved in the seven-week-long uprising renewed their cries for the world to join them in calling for embattled President Bashar Assad to give up power.
"The Syrian people are being killed and Bashar knows that he has a free hand. Nobody is really stopping him," a 28-year-old Syrian from the besieged seaside city of Banias said, asking that his name not be used out of fear for his own safety.
Assad is determined to crush the uprising despite international pressure and sanctions from Europe and the United States. European countries summoned Syrian ambassadors on Wednesday to threaten a new round of sanctions if the regime fails to halt the bloodshed.
But rights activists brushed off the threats as ineffective, saying the death toll already has exceeded those seen during the recent uprisings in Yemen and Tunisia.
"It is clear the international community is still giving the regime chances," said Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights.
In Washington, White House press secretary Jay Carney condemned the violence. "The Syrian government continues to follow the lead of its Iranian ally in resorting to brute force and flagrant violations of human rights and suppressing peaceful protests," he said, "and history is not on the side of this kind of action."
State Department spokesman Mark Toner called the Syrian attacks "barbaric," adding, "We don't throw the word 'barbaric' around here very often."
More than 750 people have been killed and thousands detained since the uprising began in mid-March, touched off by the arrest of teenagers who scrawled anti-regime graffiti on a wall.