Tanks rumbled into several districts of the central industrial city of Homs and deployed along the corniche in the northwestern coastal city of Banias, activists said.
More than 250 people were arrested in Banias, including protest leaders and doctors, the London-based Syrian Observatory for Human Rights said in a statement received by AFP in Nicosia.
Judicial authorities, meanwhile, charged prominent dissident and former MP Riad Seif, a 64-year-old who suffers from cancer, with violating a ban on protests, his lawyer Khalil Maatuk said.
The military on Saturday night cut electricity and communications before entering several Homs districts that are home to opponents of Assad's regime, a day after having taken up positions inside the city, an activist said.
Gunfire reverberated in Bab Baba and Sebaa Amr, two neighbourhoods in the city of one million inhabitants that has been the scene of almost daily demonstrations since protests in Syria erupted mid-March.
A video posted online at YouTube -- which could not be authenticated -- showed around 20 truckloads of soldiers heading into the night towards a Homs district.
Qassem Zuheir al-Ahmad, 12, was killed by gunfire in the city, where other people also died, said an activist who was unable to specify who shot the boy or give an overall casualty toll.
"Snipers are posted on rooftops," the activist said.
The military conducted a similar operation after cutting electricity, communications and water in the Mediterranean port city of Banias, said Rami Abdel Rahman of the Syrian Observatory.
"The city is isolated from the rest of the world," Abdel Rahman said and warned against "a humanitarian catastrophe in the southern districts" of Banias, where 20,000 people live. The city has a total population of 50,000.
"Tanks have been positioned on the corniche and in the southern neighbourhoods of Banias and people have been arrested," he said, adding that security forces had "lists of names" of people they want detained.
Sheikh Anas al-Ayrout, a Muslim cleric considered the head of the dissent movement in Banias, was among the more than 250 people arrested in the city, where dozens of women and doctors were also detained, the Observatory said.
Al-Watan newspaper, which is close to the government, said the army was locked "in a fierce battle against groups using heavy weapons, anti-tank rockets and machine guns" in and around Banias since Friday night.
At least six people were killed in the city on Saturday, four of them women who were protesting for the release of prisoners taken into custody by security forces, an activist said.
The military launched its action in Banias and Homs after ending a 10-day lockdown in which dozens were killed and scores detained in the southern town of Daraa, the epicentre of the pro-democracy protests that erupted on March 15.
On Sunday, a military spokesman said "units of the army and security forces continue to pursue armed terrorist groups... in Homs, Banias and the outskirts of Daraa," the state news agency SANA reported.
"Six soldiers, three of them officers, were killed in the clashes and others wounded," the spokesman said, adding that several suspects were killed or wounded and dozens arrested.
SANA also reported that 10 Syrian labourers returning home from Lebanon were killed in an ambush at dawn on Sunday near Homs.
Al-Watan said Assad met Saturday with a delegation of youths "who spoke of the violent practices of some security forces.
"Assad did not deny such practices and said it was down to individual behaviour, and that the government was working to contain the crisis and avert violence," it reported.
A member of Syria's banned Socialist Union party told AFP in Beirut he had fled his home outside Damascus after being told by other activists that he would be arrested.
"Our society is dominated by fear," Mujab Assamara, 33, said. "The revolution is extraordinary. It is for liberty and dignity... and it will be very difficult for the Syrian people to turn back."
Rights groups say more than 600 people have been killed and 8,000 jailed or gone missing in the crackdown on protesters since mid-March. The Committee of the Martyrs of the 15 March Revolution puts the death toll at 708.
The United States has warned it would take "additional steps" against Syria if it continues its brutal crackdown while the European Union decided Friday to impose sanctions on 13 Syrian officials involved in the brutal crackdown.
On Monday, EU ambassadors are due to meet to discuss whether to also target Assad himself, diplomats said.
-------------------------------------------------------------------------------------