Victims of an attack in northern Syria are believed to have been exposed to a chemical weapon, doctors and world leaders have said.

Syria denies being behind the 4 April attack in Khan Sheikhoun, a rebel-held town in Idlib province in northern Syria.

The US believes sarin gas was dropped by Syrian aircraft. Turkey says post-mortems performed on victims of the attack show chemical weapons were used.

:: The UN agency World Health Organisation said:

"The likelihood of exposure to a chemical attack is amplified by an apparent lack of external injuries reported in cases showing a rapid onset of similar symptoms, including acute respiratory distress as the main cause of death."


"Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents."

:: The charity Médecins Sans Frontières/Doctor Without Borders said:

"A Médecins Sans Frontières medical team providing support to the emergency department of Bab Al Hawa hospital, in Syria's Idlib province, has confirmed that patients' symptoms are consistent with exposure to a neurotoxic agent such as sarin gas."

"Eight patients showed symptoms - including constricted pupils, muscle spasms and involuntary defecation - which are consistent with exposure to a neurotoxic agent such as sarin gas or similar compounds."

Image: A man collects the body of a dead bird, reportedly killed in the chemical attack

:: What is sarin gas?

Sarin is a clear, colourless and tasteless liquid that has no odour in its pure form.

It is made up of four common chemical compounds: dimethyl methylphosphonate, phosphorus trichloride, sodium fluoride and alcohol.

Sarin is man-made and part of a class of chemical weapons called G-series nerve agents developed by German scientists during World War Two. Other agents include tabun, soman and cyclosarin.

At room temperature, G-series nerve agents are volatile liquids, with sarin being the worst.

Distressing images: Children killed in Syria 'chemical attack'

:: How does sarin gas kill?

Sarin interferes with the neurotransmitter acetylcholine, which acts as an off switch for glands and muscles. Blocking that switch leads to over-stimulation of muscles.

The extent of poisoning depends on how much chemical a person is exposed to and for how long.

Large doses of sarin can cause loss of consciousness, convulsions, paralysis and respiratory failure and death.

Because sarin evaporates quickly, it presents an immediate but short-lived threat.

:: What is chlorine gas?

Chlorine in liquid or tablet form is found in some household products and often used to disinfect swimming pools. But it is sometimes in the form of a poisonous gas, which is a greenish-yellow colour.

It is very reactive with the human body and very toxic.

It was used as an agent of warfare by Germany during the First World War.

Chlorine remains the simplest chemical weapon and reappeared on the battlefield during the Iraq War in 2007 and allegedly in Syria.

Evidence of Syria attack 'points to Assad'

:: How does chlorine gas kill?

Chlorine gas can react with water in the lining of lungs and form hydrochloric acid.

This destroys the lungs' ability to take in oxygen. People can die from lack of oxygen when enough gas is used.

:: So how is chlorine safely used in swimming pools?

The quantity of water in a pool is so large that the chlorine is diluted to the point where it is only very mildly toxic.

If people swim a lot with your eyes open, their vision can become blurry for a few hours afterwards.

:: Sky News will have special coverage on Syria from 3pm to 5pm