BEIRUT, Lebanon - The gunmen walked into an apartment building before dawn earlier this month in the quiet Damascus suburb of Jaramana, went to the fifth floor and knocked on the door. When the police commander opened up, the men shot him dead and left.

Syrian President Bashar al-Assad's opponents appear to be resorting increasingly to assassinations of loyalist military officers in an escalation of their campaign to bring down the regime. At least 10 senior officers, including several generals, have been gunned down in the last three months, many of them as they left their homes in the morning to head to their posts.

The latest occurred Tuesday, when attackers shot and killed a retired lieutenant colonel and his brother, a chief warrant officer, at a home supplies store in another suburb of the capital, according to the state news agency. Elsewhere in Damascus, an intelligence officer was killed, opposition activists said.

Such targeted slayings are rising as an intensified crackdown by regime forces in recent months has dealt heavy setbacks to Syria's rebels. For the moment, Assad's troops have shattered the rebels' strategy of trying to seize ground in several cities and provinces.

Their pace appears to have accelerated even more sharply since a cease-fire plan brokered by U.N. and Arab League envoy Kofi Annan went into effect April 12 - and just as quickly began to unravel.

The peace plan was meant to halt 13 months of violence by government forces to put down an anti-Assad uprising in which the United Nations says more than 9,000 people have died. A spokesman for Annan said in Geneva that satellite imagery and other credible reports show Syria has failed to withdraw all its heavy weapons from populated areas as required by the cease-fire deal.

It remains murky whether the recent slayings are being carried out by rogue elements in the opposition seeking revenge or whether they represent a coordinated strategy by rebels to destabilize the regime. A spokesman for the Free Syrian Army, the Turkish-based umbrella group for armed opposition groups in Syria, denied it was behind the string of attacks, although he said the victims were legitimate targets.

There is also a sectarian tone to the killings. Almost all the slain officers come from the religious minorities that have been the most die-hard supporters of Assad in the face of the Sunni Muslim-led uprising against his rule. Such minorities - particularly Alawites, followers of a Shiite offshoot sect - make up the backbone of the military's officer ranks.

Mohamad Bazzi, a Syria expert at the Council on Foreign Relations, said the killings are "likely a tactic by the Syrian rebels who are fighting from a much weaker position."

"In many ways, it's a classic guerrilla tactic - to strike at weak points in the regime's military and security apparatus," he said.