A history of chemical weapons in Syria

— -- The death toll from Tuesday’s chemical weapons attack on a Syrian town continues to climb amid harrowing testimonies from residents, doctors and activists at the scene.

Questions are mounting about who is responsible, with some fingers pointing to the Syrian regime.

Here’s a look at Syria's fraught history with chemical weapons and how countries around the world have agreed to chemically disarm.

The genesis of modern chemical warfare

Chemicals have been used as tools of war for hundreds of years, in the form of poisoned arrows and toxic fumes. But today’s deadly chemical warfare originates from the battlefields of World War I.

The first large-scale deployment of chemical warfare agents occurred in Belgium in April 1915 during World War I when the German military unleashed green clouds of chlorine gas on French Algerian troops.

By the end of the war, some 124,000 tons of chemical agents had been used, resulting in an estimated 100,000 deaths and over one million casualties, according to the Organization for the Prohibition of Chemical Weapons (OPCW).

The horror of the gas attacks during World War I led to the 1925 Geneva Protocol, which banned the use of chemical and biological weapons in war.

The OPCW was formed in 1997 to implement the Chemical Weapons Convention, an international treaty against the use, development, production, stockpiling and transfer of chemical weapons and their precursors. Any chemical used for warfare is deemed a chemical weapon by the convention. All states who have signed the treaty have agreed to chemically disarm by destroying any of their stockpiles of chemical weapons, as well as any facilities that produce them.

As of March 2016, 192 states have signed and ratified the Chemical Weapons Convention. Israel has signed, but not ratified the treaty, while Egypt, North Korea and South Sudan have neither signed nor ratified the agreement, according to a fact sheet from the OPCW.

Syria signed the Chemical Weapons Convention in 2013 and submitted to inspections and removal of what it said were stores of chemical weapons in 2014, though opposition groups maintained they had not given a full account.

The OPCW says that nearly 95 percent of the world’s declared stockpile of chemical weapons have been destroyed under their verification.

A history of chemical weapons use in Syria

Reports of the first mass use of chemical weapons by Syrian President Bashar al-Assad’s regime began four years ago amid the country’s ongoing conflict.

In March 2013, both sides in the Syrian Civil War traded accusations over a gas attack that killed dozens of people, including government soldiers, in Khan al-Assal, a district of Aleppo city in northern Syria. An investigation by the United Nations later concluded that sarin nerve gas was used in the attack, but the international body did not identify a culprit.

In August 2013, Syrian government forces were accused of using chemical weapons against rebel-held suburbs outside the capital of Damascus. More than 1,400 people were estimated to have suffocated to death in Ghouta, with many suffering symptoms such as convulsions, constricted pupils and foaming at the nose and mouth and rapid heartbeat, according to a U.S. intelligence report.

Syria acknowledged possession of chemical weapons in September 2013, after the U.N. Security Council ordered the regime to account for and destroy its chemical weapons stockpile.

The persistent reports of chemical weapon attacks prompted the OPCW in 2014 to set up a mission "to establish facts surrounding allegations of the use of toxic chemicals, reportedly chlorine, for hostile purposes" in Syria.

The OPCW’s so-called fact-finding mission was deployed to areas inside and outside Syria to interview witnesses and obtain samples and physical evidence for analyses. Its findings confirmed that a toxic chemical had been used "systematically and repeatedly" as a weapon in villages in northern Syria.

U.N. investigators also concluded in 2014 that "chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians, including children, on a relatively large scale."

"This is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja (Iraq) in 1988," then U.N. Secretary-General Ban Ki-moon said at the time. "The international community has pledged to prevent any such horror from recurring, yet it has happened again."

The OPCW subsequently ordered Syria to destroy all chemical weapons material and equipment by the first half of 2014. Syria provided the international organization with an inventory of its chemical weapons arsenal and began eliminating them in October 2013.

The OPCW announced in August 2014 that all Category 1 materials, which pose a "high risk" to the Chemical Weapons Convention and include the nerve agents VX and sarin, declared by Syria had been destroyed.

However, there have been sporadic reports of the further use of chemical weapons by both parties in the Syrian Civil War, leading up to the latest chemical attack.

At least 86 civilians, including 30 children and 20 women, have died from the attack in the town of Khan Sheikhoun, according to the Syrian Observatory for Human Rights.

Turkish Justice Minister Bekir Bozdag told reporters on Thursday that autopsy results performed on the bodies of three people who were killed revealed that chemical weapons were used in the attack in Syria’s Idlib province. Bozdag didn’t provide further details and he didn’t specify what type of chemical was used in the attack.

Thirty-two victims injured in the attack were brought across the border into southern Turkey for treatment. Three of them have since died, according to the Turkish justice minister.

U.S. intelligence officials, the World Health Organization and Doctors Without Borders have all said the attack may have involved a banned nerve agent.

Russia on Wednesday blamed Syrian rebels for the attack, saying that the Syrian Air Force struck a warehouse where opposition militants were storing chemical weapons -- a statement that contradicts testimonies from residents, doctors and White Helmets on the ground.

U.S. Secretary of State Rex Tillerson released a statement on Tuesday condemning the attack while placing the blame on Assad.

"While we continue to monitor the terrible situation, it is clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism," Tillerson said in the statement. "Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions. Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable."

Tillerson also called upon Russia and Iran to "exercise their influence over the Syrian regime and to guarantee that this sort of horrific attack never happens again."

Tuesday's attack is the latest atrocity in Syria's ruinous six-year war.

What started as a local protest movement in Syria’s southern city of Dara’a expanded into a full-fledged civil war by 2012. ISIS, which grew out of al-Qaeda in Iraq, took root in northern and eastern Syria in 2013 after seizing swaths of territory in neighboring Iraq. The jihadist group is fighting to overthrow Assad’s regime and establish an Islamic caliphate.

The civil war has since burgeoned into today’s brutal conflict, pulling in the United States, Russia, Iran and almost all of Syria’s neighbors. It has caused the largest humanitarian crisis since World War II, according to the U.N.

ABC News’ Rachel Katz, Luis Martinez, Lena Masri, Rym Momtaz, Darren Reynolds, Joseph Simonetti, Bianca Seidman and Marcus Wilford contributed to this report. The Associated Press also contributed to this report.