Syrian President Bashar Assad has denied that his forces have used so-called "barrel bombs" in the country's bloody civil war. But what is a barrel bomb and why is it such a controversial method of warfare?

Barrel bombs are crudely made and kill indiscriminately

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A barrel bomb can refer to any large container packed with gasoline, nails and chunks of steel that is typically thrown out of a helicopter. These improvised explosive devices represent a cheap form of aerial warfare — but their rudimentary and unguided design means they can kill civilians through inaccuracy. Describing their use in Syria, the then State Department Spokeswoman Victoria Nuland described barrel bombs in 2012 as "vicious things indiscriminately launched ... at targets without any concern about civilians."

Assad has been widely accused of using barrel bombs

"They're called bombs. We have bombs, missiles and bullets," Assad said during the rare interview with the BBC that aired Tuesday. "There [are] no barrel bombs, we don't have barrels. I haven't heard of the army using barrels, or maybe, cooking pots." Jeremy Bowen, the veteran BBC journalist who interviewed Assad, described this response as "flippant" and said barrel bombs were "the most notorious weapon in the regime's arsenal." This view is shared by the U.S. and its allies, noting that only the Assad regime uses helicopters —the typical method of dropping barrel bombs — in Syria. "Each and every day that the barrel-bombing of Aleppo continues, the Assad regime reminds the world of its true colors," Secretary of State John Kerry said last February.

The U.N. has unsuccessfully demanded the outlaw of barrel bombs in Syria

In a resolution passed unanimously last year, the United Nations Security Council called for a halt to the use of barrel bombs in Syria. The resolution said the bombs "cause superfluous injury or unnecessary suffering" and called for all parties to "show respect for international humanitarian law." However the activist group Human Rights Watch said in June that the Syrian government had defied this resolution. "By using barrel bombs on densely populated areas, Syrian government forces are using means and methods of warfare that do not distinguish between civilians," it said.

Barrel bombs have killed thousands of people

Some 200,000 people have been killed in the five-year conflict between Assad's government and rebel groups. It is harder to pinpoint the exact number of people killed by barrel bombs, but rebel activists say it is in the thousands. Their use has been particularly prevalent in rebel held areas of Aleppo and Damascus, U.S. special representative to the U.N. Samantha Power told the Security Council last month. The Assad regime has also been dropped "barrel bombs on medical facilities as if they were military encampments," said Terri Robl, U.S. Deputy Representative to the UN Economic and Social Council, in December.