Syrian troops stormed Hama following large protests in the flashpoint central city on Tuesday, as three people died in violence elsewhere in the country, activists said.
"The Syrian armed forces stormed the neighbourhoods of Bab Qubli and Al-Jarajmah in Hama, firing heavy machineguns," the Syrian Observatory for Human Rights said.
Activists in the city said that troops had gone on the offensive after large demonstrations earlier in the day.
"Since the morning, entrances to Hama have been blocked... Syrian troops stormed the city from its northern and western entrances," Saleh al-Hamwi, spokesman of the General Revolution Commission, told AFP by telephone.
Anwar Amran, another anti-regime activist in Hama, said tanks had entered the city and there had been "heavy machinegun fire" in three different neighbourhoods.
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The London-based Observatory said two civilians were killed in Hama province and one in Homs, Syria's third-largest city.
"A civilian was killed by gunfire from a checkpoint... in the Bab al-Sabaa neighbourhood of Homs," one of the focal points of protests against President Bashar al-Assad's regime that erupted in March last year, it said.
Another two civilians died in Hama province when security forces fired on the car they were travelling in near the Tibah al-Imam locality.
Separately, Syrian security forces reportedly opened fire in Idlib province in the northwest, in an attempt to disperse some 10,000 people gathered for the funeral of Radwan Rabi Hamada, an anti-regime protester killed in the town of Saraqeb on Monday.
Observatory chairman Rami Abdel Rahman said security forces fired heavy machineguns after Hamada's burial in the village of Al-Bara.
And in the southern city of Daraa, clashes broke out between deserters and the regular army, the watchdog added.
The United Nations estimates that more than 5,400 people have been killed in Syria since last March.