The lethal attack Wednesday on President Bashar Al-Assad’s senior security chiefs aligned neatly with a tactical shift that had changed the direction of Syria’s long conflict: the opposition fighters’ swift and successful adoption of makeshift bombs.

Bombs have been in rebel use since violence intensified in Syria in late 2011. But since mid-spring, anti-Assad fighters have become bolder and sharply more effective with their use, and not only in what is apparently their hand in the assassinations in Damascus.

Improvised bombs have steadily become the most punishing weapon in the otherwise underequipped rebels’ arsenal, repeatedly destroying Syria’s main battle tanks, halting army convoys and inflicting heavy casualties on government ground operations in areas where armed resistance is strong, Western analysts and rebel field commanders and fighters said.

In this way, even as the anti-Assad fighters have appealed for international intervention and other forms of material and military support, local fighters have created their own informal buffer zones, pockets of the Syrian countryside that are now largely free of government ground troops.

“The bomb is not only essential, it is a main part of our success,” said a former Syrian Army artillery major, who called himself Abu Akhmed and leads a fighting group in Idlib, a northern Syrian province, in a meeting in a house in this Turkish city crowded with fighters.

“When you think of why we are improving and getting stronger, it is not because more weapons are coming in from outside,” he added. “The main reason is because we are becoming more organized, and because of our bombs.”

The bombs that Abu Akhmed described, known in Western military jargon as improvised explosive devices, or I.E.D.’s, have done more than kill Syrian soldiers and deny the Syrian Army access to Syrian terrain. The weapon that has long been championed in the popular imagination and public discourse of underground fighters as a means to kill or drive off foreign occupiers — whether Russians in Chechnya or Americans in Iraq and Afghanistan — has been turned against a standing Arab army by its own people.

The shift happened subtly. Joseph Holliday, a former American Army intelligence officer who is now an analyst covering Syria for the Institute of the Study of War, in Washington, said the changes were not in the rate of attacks, but in a rapidly evolving prowess.

“The percentage of I.E.D. attacks compared to overall rebel activity has not increased in a statistically significant way,” Holliday wrote by e-mail, just hours before the assassinations in Damascus. “What has increased is the percentage of effective attacks.”

But, he added, “what has increased the most, and this has been the hardest thing to put a finger on through open source research, is the number of what US military might call ‘catastrophic’ I.E.D. attacks.”

By that he meant bombs that destroyed heavily armored tanks, or caused large numbers of casualties.

Although precise casualty estimates are impossible to obtain, one senior Obama administration official who spoke on the condition of anonymity under administration practice said that by June the Syrian military was suffering an average of 20 dead soldiers a day in various types of attacks, and several times that number of wounded. This would be a significant drain on a force already suffering from defections and now trying to suppress an escalating guerrilla conflict by conventional means.

The exact means by which anti-Assad fighters have improved their manufacture and use of bombs, and who trained them, is not clear.

Holliday said the capability “comes in part from the expertise of Syrian insurgents who learned bomb-making while fighting US troops in eastern Iraq.”

A US official who follows the fighting in Syria and spoke on the condition of anonymity noted another example of turnabout. Some of the expertise, the official said, appeared to have been derived from the very trainers in explosives, who were formerly in Syrian intelligence or under its tutelage, with which Syria for decades exported bomb-making and other lethal skills to groups it sponsored in neighboring states.

Sensitive to being compared to groups that have detonated bombs indiscriminately, Syrian opposition fighters said that they were fighters using the only tools they had to succeed, and that they aimed at only military targets. They said the uptick in their success was a result not of outside help, but of local trial and error, and in some cases, they were forced into the experience by economics.

With prices of guns and ammunition soaring, they noted, bombs could be made with cheaper materials, giving fighters with limited means or limited access to traditional infantry arms an inexpensive way to fight.

Another commander said he bought blasting caps and then made the main explosive charges himself using urea-based fertilizer, sugar and sulfur, among other things, part of recipes fighters took from the Internet and circulated between rebel fighting groups. It took time, he said, to master the skill: “Once when I was making a bomb, I burned part of my house.”

The fighters also said they had learned many ways to explode their bombs, including via a direct electrical wire connection or remotely via a radio or cellphone signal.

Anti-Assad commanders credited the bombs with helping to change the fighters’ psychological experience of the battle against their government.

Many rebel fighters, who said they were once afraid of government forces, now said they saw government ground operations as opportunities to kill Syrian troops along the roads, weaken the government and frustrate the army — a shift that emboldened them and engendered confidence.