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There also have been increasing reports of attacks on security forces and police patrols. Mohammed Saleh, an opposition figure in Homs, said he saw a police car riddled with bullets and a burned-out security bus in the city last week.

Mr. Saleh also said gunmen attacked an army force last week, destroying six armored personnel carriers in Homs.

Another Homs activist, Majd Saleh, confirmed clashes between Syrian troops and army defectors.

“The opposition to the government is gradually transforming into more of an armed resistance,” said Vali Nasr, a Middle East expert and former Obama administration official in the State Department.

“The brutality of the regime has become enormous, and there is increasing pressure on people to defend their families and their villages. They clearly have won a moral argument against the government, but physically it doesn’t protect them.”

There is no central call to arms by the opposition, in part because there is no clear leadership in the movement. The Syrian opposition is disparate and fragmented, with various parties vying for power as they see an end to more than 40 years of iron rule by Mr. Assad and his late father, Hafez.

Still, recent weeks have seen a subtle change in tone. Some Syrians are calling on protesters to take up arms and inviting foreign military action, hoisting signs that say, “Where is NATO?” and urging the world to come to Syria’s aid.The U.N. human rights office says the regime has started retaliating against protesters’ families to snuff out the uprising, a fearsome new tactic that could be having a chilling effect on the revolution.

“Prominent human rights defenders, inside and outside the country, are reported to have been targeted,” U.N. human rights office spokeswoman Ravina Shamdasani said in Geneva on Friday. “We are also concerned by reports of the targeting and attacking of families and sympathizers of the protesters by security forces.”

There is little sign of the violence ending anytime soon. International intervention, like the NATO action that helped topple Col. Gadhafi, is all but out of the question. Washington and its allies have shown little appetite for intervening in yet another Arab nation in turmoil.

International sanctions, some of which target Mr. Assad personally, have failed to persuade him to ease his crackdown. There had been hopes, since dashed, that European Union sanctions would prove a humiliating personal blow to Mr. Assad, a 46-year-old eye doctor who trained in Britain.