Described as “flying IEDs” — or improvised explosive devices — barrel bombs have the power to wipe out a row of buildings in a single blast and can kill large numbers of people, including unintended victims.

“It’s fair to say that a lot of governments are losing control of the counterinsurgency,” said Michael Knights, an analyst at the Washington Institute for Near East Policy. “They’re also watching what they see in Syria, and they feel like their air power is what is making the difference.”

“Barrel bomb” is a catchall term for a large container packed with fuel, chemicals or explosives and often scraps of metal that, in recent years, have most often been dropped on targets from helicopters or planes overhead. However, they also have been found on Israeli beaches, where authorities believe they washed up after militants on the Gaza Strip released them.

They are attractive to governments that have the aircraft to bombard targets from the skies but limited munitions or money to launch enough conventional weapons, like missiles, to rival their enemies.