(CNN) These methods of warfare are so grotesque, they're banned by international law.

Airstrikes on civilian hospitals. Barrel bombs that kill and maim indiscriminately. Chemical attacks that suffocate children with their eyes still open.

But these are the threats Syrian civilians face every day. An estimated 90,000 to 120,000 civilians have been killed since the war began six years ago.

What's the motivation for killing civilians?

Civil defense workers try to reduce the effects of toxic gas as they carry out search-and-rescue missions in Khan Sheikhoun.

Civil defense workers try to reduce the effects of toxic gas as they carry out search-and-rescue missions in Khan Sheikhoun.

This photo provided by the Idlib Media Center shows victims of the suspected attack.

This photo provided by the Idlib Media Center shows victims of the suspected attack.

Al-Jundi described how whole families were killed: They died of asphyxiation, and foam covered their mouths. Many died suddenly. "I believe this horrible memory will stay with me for the rest of my life," al-Jundi said.

Al-Jundi described how whole families were killed: They died of asphyxiation, and foam covered their mouths. Many died suddenly. "I believe this horrible memory will stay with me for the rest of my life," al-Jundi said.

A dead baby is seen in the aftermath of the attack. "I've never seen anything like it, beyond description," said Fares al-Jundi, a doctor who spoke with CNN. He lives in a nearby village and rushed to the hospital after the airstrikes.

A dead baby is seen in the aftermath of the attack. "I've never seen anything like it, beyond description," said Fares al-Jundi, a doctor who spoke with CNN. He lives in a nearby village and rushed to the hospital after the airstrikes.

A baby's dead body is covered. Many of the casualties came as a result of asphyxiation, local doctors say.

A baby's dead body is covered. Many of the casualties came as a result of asphyxiation, local doctors say.

In this photo provided by the Idlib Media Center, doctors treat a child after the suspected attack.

In this photo provided by the Idlib Media Center, doctors treat a child after the suspected attack.

A man carries a child at a makeshift hospital in Khan Sheikhoun. Activists have blamed the Syrian regime for the attack. In an apparent reference to the attack, two state-run Syrian media outlets reported that there was an explosion at a "rebel poison gas factory" in the countryside of Idlib province. The government has denied using chemical weapons in the past.

A man carries a child at a makeshift hospital in Khan Sheikhoun. Activists have blamed the Syrian regime for the attack. In an apparent reference to the attack, two state-run Syrian media outlets reported that there was an explosion at a "rebel poison gas factory" in the countryside of Idlib province. The government has denied using chemical weapons in the past.

This photo, provided Tuesday, April 4, by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun, Syria. Dozens of people were killed, according to multiple activist groups.

This photo, provided Tuesday, April 4, by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun, Syria. Dozens of people were killed, according to multiple activist groups.

The civil war started after President Bashar al-Assad's regime violently cracked down on peaceful protesters, and ordinary citizens took up arms to try to oust his dynastic regime.

So from the beginning, Assad's enemy was opposition members in his own country. In other words, rebels who had started out as civilians. And that's why civilians are at risk.

"In general, the regime's strategy has been to inflict mass punishment against opposition-supported populations or populations that were perceived to be supportive of the opposition in order to prevent the formation of a viable alternative to the regime," said Chris Kozak, Syria research analyst at the Institute for the Study of War.

What are some of the ways civilians are killed?

The methods are as diverse as they are horrific. Here are a few:

Chemical weapons

The 86 people killed in a chemical attack in Khan Sheikhoun this week were likely exposed to neurotoxic agents, the international aid group Medecins Sans Frontieres (Doctors Without Borders) said.

The government said a "poison gas factory" operated by terrorists had exploded. But survivors and witnesses described a much different scene -- chemical bombs falling from the sky

US military and intelligence officials said Syrian planes dropped bombs at the same time and place that the chemical attack reportedly happened. Officials said they followed the radar track of regime planes and the infrared heat signature of the bombs.

But Tuesday's attack wasn't the first chemical attack that killed Syrians, nor was it the deadliest. In 2013, another chemical attack in Syria killed about 1,400 people . A UN report found that sarin gas had been used on civilians in that attack.

Airstrikes on hospitals

Civilian hospitals are supposed to be safe from attacks during war. Bombing them would be a clear violation of international law

But the regime has repeatedly committed that war crime, medical experts and Syrian activists have said.

The largest surgical hospital in Syria's largest city was bombed twice in four days last year. Aleppo's M10 hospital was struck by a "torrent" of weapons, including two barrel bombs, two cluster bombs and at least one rocket , a spokesman for the Syrian American Medical Society said last October.

That same week, another Aleppo hospital was struck.

"Indiscriminate bombing and shelling continues in a shocking and unrelenting manner, killing and maiming civilians, subjecting them to a level of savagery that no human should have to endure," United Nations aid chief Stephen O'Brien said after those hospital attacks.

So wounded civilians who managed to survive attacks in their neighborhoods risk their lives just by seeking medical treatment.

Barrel bombs

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They are one of the most gruesome ways of killing and maiming civilians.

These bombs typically consist of barrels stuffed with explosives and objects, such as nails or shrapnel, to maximize carnage after the barrel explodes.

The crude weapons are typically dropped from helicopters and are banned by international law because they kill and maim indiscriminately.

Why launch a chemical attack in Khan Sheikhoun, and why now?

Khan Sheikhoun is a key opposition town located on a crucial supply route on major highway.

"This town is a major funneling route to send supplies to Hama province from southern Idlib (province)," Kozak said.

He said the timing of Tuesday's chemical attack makes sense, too. For the past several weeks rebels have been attacking regime-held Hama city, just up the highway.

"I think this town was on the target list by the government." Kozak said.

But not everyone believes Assad is behind that chemical attack. US House Rep. Thomas Massie is skeptical.

"Frankly, I don't think Assad would have done that," said Massie, who voted against intervention in Syria in 2014. "It does not serve his interest. It would tend to draw us into the civil war even further."

How does the regime respond to claims it's killing civilians?

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The Syrian regime has long insisted it's not targeting innocent people. Since 2011, the government says it has been fighting "terrorists" -- long before terrorists from ISIS starting making headway in the war-torn country.

That's not to say rebels are without blame. They, too, have been accused of war crimes.

"There's been plenty of shelling by rebel forces on regime-held parts of Aleppo and Damascus -- they've conducted indiscriminate attacks," Kozak said. But those "pale in comparison to the scale of attacks against civilians" committed by Assad's government, he added.

"The regime remains responsible for the majority of civilian deaths in the Syrian civil war" he said.

Why does the regime get blamed for most of the civilian deaths?

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Simply put, rebels and other factions don't have the equipment or firepower to launch many of the major attacks that have killed civilians. Kozak calls this "asymmetrical warfare."

"Asymmetrical warfare is basically when war is conducted by fighters that are not of equal strength," he said. "The regime has a standing military, (and) the opposition does not."

And that gives the regime a technological edge.

"They can use the air force, ballistic missiles, chemical weapons, mass heavy artillery," Kozak said.

Are attacks against civilians helping either side in the war?

The Assad government certainly benefits from eliminating civilians in opposition areas, Kozak said.

The regime has managed to "besiege, soften, and ultimately force the surrender of opposition-held urban centers -- thereby avoiding manpower-intensive urban warfare," he said.

And the attacks can "demoralize and displace civilian populations in opposition-held terrain."

"The greatest threat facing the regime is ultimately the creation of a credible political alternative that could rally international support and challenge its claims to legitimate governance," Kozak said.

"The regime's sustained pressure on civilian populations and infrastructure precludes this development by disrupting day-to-day life in opposition-held regions and driving civilians to flee either into regime-held terrain or out of the country entirely. The regime thus bolsters its own claim as the sole legitimate protector of its citizens in part through a humanitarian crisis of its own design."