Syrian security forces pushed into the city of Hama on Tuesday, witnesses said, to quell violence the government is blaming on armed gangs.
State television broadcast footage purporting to show protesters firing weapons at Syrian troops.
Rights campaigners say 134 people have died in Syria over the past three days, 90 of them in Hama.
Reports from the country are hard to independently verify since Damascus has severely restricted the movement
The United States and Britain insist military action is not an option.
Both have urged President Bashar-al Assad to accept the protesters’ demands for reform or step down.
British Foreign Secretary William Hague said intervention was “not a remote possibility”.
He said it was important to “persuade President Assad to reform instead of embarking on these appalling actions”.
The recent violence in Hama has evoked memories of 1982 when Hafez al-Assad, Bashar’s father and predecessor, crushed a revolt by Sunni opposition group the Muslim Brotherhood.
That crackdown left tens of thousands of people dead.