(CNN) Horrifying images and video show civilians -- including children -- struggling to breathe, foam coming from their mouths as they appear to die of asphyxiation after an airstrike on a rebel-held town in northwestern Syria on Tuesday.

Syria's military has denied using chemical weapons and blamed rebels for the carnage , but activists say the regime is responsible for a chemical attack which killed at least 86 people. Russia, meanwhile, has blamed the deaths on a Syrian airstrike on a "terrorist" ammunition depot.

It's not yet confirmed what chemical agent was involved in the suspected attack in Khan Sheikhoun, Idlib province. But early indicators point to the release of a nerve agent like Sarin

"The symptoms were pale skin, sweating, narrow or pin-eye pupils, very intense respiratory detachments. Those symptoms match the usage of Sarin," said a doctor in a hospital close to Khan Sheikhoun, who cannot be named for security reasons.

Does the Syrian regime have nerve agents?

The regime of Syrian President Bashar al-Assad is supposed to have given up its chemical weapons stockpile, but not everyone is convinced.

Before the war began in 2011, Syria had research and production facilities near Damascus, Aleppo, Homs, Latakia and Hama that turned out hundreds of tons of chemical agents a year, according to the US-based James Martin Center for Nonproliferation Studies and the Nuclear Threat Initiative, which catalogs the world's arsenals of weapons of mass destruction.

Syria never signed the Chemical Weapons Convention, the current international treaty against the use of poison gas, but it did sign the 1925 Geneva Protocol, which banned the use of chemical and bacteriological warfare, according to the international Organisation for the Prohibition of Chemical Weapons (OPCW.)

Experts from the agency were part of a UN team that found "clear and convincing evidence" that Sarin was delivered by surface-to-surface rockets "on a relatively large scale" in the Ghouta area of the Syrian capital Damascus in August 2013.

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From October 2013, the OPCW oversaw the destruction of Syria's chemical weapon stockpile. But one Syrian general, who said he defected after he was ordered to use chemical agents, told CNN's Christiane Amanpour at the time that the regime would never give up its chemical stockpile.

"The locations of most of the scientific research centers in Syria and the storage facilities are known and under surveillance, thus, he will give up those centers and facilities for sure without lying. That said, however, Bashar al-Assad will not give up the chemical stockpile," said Brig. Gen. Zaher al-Sakat in 2013.

Could rebel forces have chemical weapons?

Russia's defense ministry claimed on its Facebook page that a Syrian airstrike had hit a "terrorist" ammunition depot in the eastern outskirts of Khan Sheikhoun.

The ministry said "terrorists had been transporting chemical munitions from this largest arsenal to the territory of Iraq."

Syrian Deputy Foreign Minister Fayssal Mikdad similarly told al-Mayadeen TV that Syria had given the OPCW and United Nations information in recent weeks on the Jabhat al-Nusra terrorist group transporting toxic substances into Syria, state-run news agency SANA reported.

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But Dan Kaszeta, a chemical weapons specialist and managing director of Strongpoint Security, told CNN that the Russian version of events was "highly implausible."

All the nerve agents used in the Syrian conflict so far have been binary nerve agents, he said, which are mixed from different components within a few days of use. This is done because of the difficulties of handling agents such as Sarin, which has a very short shelf life, he said.

"Nerve agents are the result of a very expensive, exotic, industrial chemical process -- these are not something you just whip up," he said.

The idea that the Syrian opposition would be able to build the covert supply chain to make a nerve agent and then would move it around and store it in a warehouse, rather than a bunker, makes no sense, Kaszeta said.

"It's much more plausible that Assad, who's used nerve agents in the past, is using them again," he said.

OPCW investigators have accused ISIS of using sulfur mustard, a blister agent, in Iraq and Syria. But Kaszeta said there was no comparison, since sulfur mustard -- widely used in Saddam Hussein's Iraq -- is much easier to come by, is far less lethal and has a long shelf life.

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Charles Lister, a senior fellow at the US-based Middle East Institute, also rejected the Russian account of what happened as "laughable."

"Are we seriously meant to believe that the opposition has a latent chemical weapons capability, and yet somehow, it has only ever suffered the effects of its own weapons and failed to use them itself?" he said.

What happened in Khan Sheikhoun "is an almost exact replication of what we saw in the summer of 2013," Lister told CNN, adding that it had targeted a key staging area for an opposition push into northern Hama late last month.

Lister also highlighted the issue that all nerve agents known to be in Syria are binary weapons.

"First of all, nobody in their right mind would ever store both both components of a binary nerve agent in the same building. And secondly, even if they were stored together and then targeted, blowing them up would not result in any active nerve agent -- it's chemically impossible," he said.

What chemical agent was involved?

This won't be known for sure until tests are carried out but medical experts report symptoms consistent with the use of nerve agents, which include Sarin.

The World Health Organization says the victims' symptoms are consistent with exposure to nerve agents, pointing to "the apparent lack of external injuries" and "acute respiratory distress as the main cause of death."

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Doctors without Borders, or MSF, said a team providing support to the emergency department of Bab Al Hawa hospital in Idlib province had "confirmed that patients' symptoms are consistent with exposure to a neurotoxic agent such as Sarin gas."

MSF teams who visited other hospitals in the area "reported that victims smelled of bleach, suggesting they had been exposed to chlorine," a news release said. "These reports strongly suggest that victims of the attack on Khan Sheikhoun were exposed to at least two different chemical agents."

A joint OPCW-UN panel found last year that Syrian government forces had used chlorine gas as a weapon. Syria denies using chemical weapons.

Khaula Sawah, board member and former CEO of the Union of Medical Care and Relief Organizations (UOSSM), said her field team in Idlib had reported that while still unconfirmed, doctors believe -- based on the symptoms of the patients -- that some form of organophosphate nerve agent was used.

A Syrian doctor treats a child at a makeshift hospital, in the town of Khan Sheikhoun, northern Idlib province, Syria, on April 4.

"The symptoms rapidly developed within minutes, including redness of eyes, foaming at the mouth, contracted pupils, severe dyspnoea (labored breathing) or shortness of breath and suffocation," she told CNN. "The symptoms progressed so rapidly that fatality occurred within 10 minutes. If there were no supportive care or ICU, people were dying."

The Syrian American Medical Society also said hospitals in the area around Khan Sheikhoun had "received patients who suffered symptoms indicative of an organophosphorus compounds agent, a category of toxic gases which includes Sarin."

So what is Sarin?

Sarin is a clear, colorless, tasteless and odorless liquid that was originally developed in 1938 in Germany as a pesticide. It was used in two terrorist attacks in Japan in 1994 and 1995.

It's less toxic than nerve agent VX but it's extremely volatile. It mixes easily with water -- symptoms can be brought on by drinking or even touching a contaminated water supply. It can also contaminate food and clothing.

Its effects depend on the degree and manner of exposure, says the US Centers for Disease Control and Prevention

In liquid form, symptoms are likely to appear a few minutes to a few hours after exposure, but the effect is almost instantaneous when it's an odorless gas. Then, victims can experience symptoms within seconds.

Symptoms of low to moderate exposure include a watery nose, blurred vision, tightness in the chest, nausea, drowsiness and headaches.

Exposure to large doses can lead to loss of consciousness, convulsions, paralysis and respiratory failure. Victims can die within 10 minutes.