[26] In August and October 2016, United Nations reports explicitly blamed the Syrian military of Bashar al-Assad for dropping chlorine bombs on the towns of Talmenes on 21 April 2014, Sarmin on 16 March 2015.

[30][31] The immediate context for the Khan Shaykhun attack was the intensified aerial campaign in March and April 2017 by the government and its Russian ally to gain control of Kafr Zeita, Murek and al-Lataminah, then the three remaining rebel-held towns in the northern Hama Governorate.

More than 70 people in the area were then exposed to an unidentified chemical agent and showed symptoms of nausea, agitation, foaming, muscle spasm, and miosis (constriction of the pupil of the eye).

[42][43][39] Other symptoms reported included coldness in the extremities, decreased heart rate, and low blood pressure,[40] convulsions, foaming at the mouth or vomit, and respiratory paralysis.

[44] On 6 April, the Turkish Ministry of Health, which had conducted tests on people transported to Turkey, said it had identified the chemical used in the attack as sarin, citing lung damage found in victims.

[45] On 11 April, Turkish Minister of Health Recep Akdağ stated that isopropyl methylphosphonic acid—a known byproduct of sarin reacting with other compounds—was "identified in the blood and urine tests conducted on samples taken from the victims".

The opposition minister of health, Mohamad Firas al-Jundi, said victims experienced suffocation, fluid in the lungs, foaming at the mouth, unconsciousness, spasm, and paralysis.

A visit by The Guardian to Khan Sheikhun two days after the attack revealed that the site officials claimed had been hit had been empty for many months, and contained only animal feed and a volleyball net.

[44][62][63][64] According to investigation by Human Rights Watch, the attack was conducted by Syrian government forces from the air using Soviet-made KhAB-250 aerial bombs designed to deliver sarin.

[44] Immediately following the attacks, Russia said the Syrian Air Force had struck Khan Shaykhun "between 11:30am and 12:30pm local time" on 4 April, but that the target had been "a large terrorist ammunition depot" on its eastern outskirts.

[79] In a 13 April interview to AFP, President Assad said the attack is "100 per cent fabrication" by the United States "working hand-in-glove with the terrorists", intended to provide a pretext for the airstrike on the Shayrat Airbase.

[70] The Russian Foreign Ministry said it was "premature to accuse the Syrian government of using chemical weapons in Idlib", and insist on full and impartial investigation.

[74][82] Shortly after the attack Russian presidential spokesman Dmitry Peskov told reporters the use of chemical weapons is a "dangerous and monstrous crime" and that Russia's support for Assad is not "unconditional".

[23] UK based Bellingcat founder Eliot Higgins observed that Russian claims that a warehouse containing chemical weapons was bombed related to a raid carried out "two to three hours" after the first images of victims appeared.

[91][92] President Donald Trump called the attack "reprehensible" and attributed it to the Syrian government, saying the act could not be ignored "by the civilized world" during his meeting with King Abdullah II of Jordan.

[95][96] U.S. Secretary of State Rex Tillerson said: "Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable.

[99] CIA Director Mike Pompeo confirmed on Thursday 13 April that his agency concluded the Syrian government was responsible for the chemical attack in Khan Shaykhun.

[106][107] On 24 April 2017, the United States Department of the Treasury imposed sanctions on 271 employees of the Syrian Scientific Studies and Research Center for their alleged role in producing chemical weapons.

[110] Federica Mogherini, the European Union's diplomatic chief, called the attack "awful" and said Bashar al-Assad's government bore "primary responsibility" for it.

"[113] Amnesty International said the evidence points to an "air-launched chemical attack",[114] while the World Health Organization said victims carried the signs of exposure to nerve agents.

Further, that the attack was possible because it drew from old Syrian stockpiles that Assad had vowed to destroy in 2013, indicating that Damascus has systematically cheated international inspectors for the past four years.

Opposition leader Jeremy Corbyn said "There should now be an immediate ceasefire and a UN-led investigation rapidly into what is a horrific and totally illegal action by somebody using chemical weapons against innocent people.

"[127] The Egyptian Foreign Ministry released a statement saying the "painful and unacceptable" images of the massacre reaffirm the necessity of reaching a political solution to end the crisis in Syria, in light of the international community decisions and Security Council Resolution 2254, as well as the Geneva Conventions.

[130] Iranian Foreign ministry spokesman Bahram Ghassemi condemned "all use of chemical weapons," but suggested the blame for the attack lay with "terrorist groups" rather than the Syrian government.

"[132] Israeli Prime Minister Benjamin Netanyahu called on the international community "to fulfill its obligation from 2013 to fully and finally remove these horrible weapons from Syria".

[133] Other countries who condemned the chemical attack include the Czech Republic,[134] Italy,[135] Pakistan,[136] Saudi Arabia,[137] Switzerland,[138] United Kingdom,[117] and the Vatican City.

[140] Australian Prime Minister Malcolm Turnbull said if al-Assad was found to be behind the attack, as the United States believe, it represented "a shocking war crime.

[143][144] Former head of the International Atomic Energy Agency (IAEA), Hans Blix, described the US retaliation as "measured" and specific,[145] but later criticized the rapid American military response.

[146] Other people who have expressed skepticism of the Syrian government being responsible for the attack include former UN weapons inspector Scott Ritter,[26][147] [148][non-primary source needed] US based weapons expert and MIT professor emeritus Theodore Postol,[149][150] former UK ambassador to Syria and director of the British Syrian Society Peter Ford,[151][152] investigative Israeli journalist Uri Avneri,[153][non-primary source needed] and investigative journalist Seymour Hersh.

[84][156] In 2019, the Princeton University-linked journal Science & Global Security, on whose editorial board Ted Postol sat,[157] intended to publish a paper titled "Computational Forensic Analysis for the Chemical Weapons Attack at Khan Sheikhoun on 4 April 2017" written by Goong Chen, Cong Gu, Postol, Alexey Sergeev, Sanyang Liu, Pengfei Yao and Marlan O. Scully.