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Russia criticised the world's chemical weapons watchdog for not sending experts to the site of an alleged chemical attack in Syria, saying it was "unacceptable to analyse events from a distance".

But the Organisation for the Prohibition of Chemical Weapons (OPCW) is in a difficult position, with its scientists and experts undertaking a dangerous mission in an ongoing war zone.

Here is a look at the OPCW:

- What is the OPCW? -

Founded in 1997, the OPCW based in The Hague oversees the application of The Chemical Weapons Convention (CWC) aimed at ridding the world of toxic arms and preventing new ones being manufactured.

Chemical weapons, such as mustard gas, were first used in combat on the battlefields of World War I, and also in 1988 by late Iraqi dictator Saddam Hussein against civilians in Halabja, Iraq. Other incidents include the 1995 subway sarin gas attack in Tokyo by a Japanese cult.

After almost 20 years of negotiations, the convention took effect on April 29, 1997. The OPCW now has 192 member nations.

In the past two decades, the OPCW has overseen the destruction of some 94 percent of the world's declared chemical weapons stockpiles. It is planning a ceremony on April 26 to mark its 20th anniversary.

Russia and the United States are on target to destroy their chemical arms stockpiles within the next seven years.

Only four nations have not yet signed up to the convention -- Egypt, Israel, North Korea and South Sudan.

- What is the OPCW mission in Syria? -

After years of denials, the Syrian regime caved to international pressure in September 2013 and agreed under a US-Russia deal to hand over its toxic stockpile to the OPCW for destruction, averting threatened US air strikes.

The admission came after a sarin gas attack in August 2013 on rebel-held areas near Damascus that was blamed by the West and the opposition on the regime. An estimated 1,000 people died.

The OPCW says 100 percent of Syria's declared chemical arms -- a total of 1,300 metric tonnes -- was handed over and destroyed by January 2016.

However, amid continuing attacks there are fears Damascus did not declare everything. The secretary general Ahmet Uzumcu has highlighted what he calls "gaps and inconsistencies" in Syria's 2013 declaration.

Chlorine, which can be used to make choking chlorine gas, is however a common chemical for such uses as fertilisers and water purification and does not need to be declared as a chemical weapon.

- How does OPCW work in Syria? -

For the first time in the OPCW's history, it is investigating chemical weapons in a country embroiled in a civil war.

In 2014, a fact-finding mission of scientists and experts was set up to investigate persistent allegations of chemical weapons attacks. It has deployed "numerous" times to Syria and uses "investigative methods to determine if chemical weapons have been used", according to the OPCW.

After an OPCW team hit a home-made roadside bomb in Syria in early 2014, experts can no longer travel outside of Damascus for their own safety.

Instead, the OPCW "interviews witnesses and obtains environmental and biomedical samples and physical evidence for analysis" in OPCW-designated labs.

The reports of the fact-finding mission are sent to a joint UN-OPCW team known as the JIM, set up by the UN Security Council in 2015 to determine "to the greatest extent feasible" who is behind the attacks.

It has so far found the Syrian military to blame for at least three chemical attacks in villages in 2014 and 2015. And it has said the so-called Islamic State group was behind a 2015 mustard gas attack.

- What about the April 4 attack? -

The OPCW has an "ongoing" investigation into the April 4 attack on the rebel-held town of Khan Sheikhun, in Idlib province, which killed 87 people.

At a special OPCW meeting in The Hague on Thursday, Uzumcu said technical experts had analysed available information "and their preliminary assessment (was) that this was a credible allegation".

Britain told the meeting its scientists had analysed samples which "have tested positive for the nerve agent sarin, or a sarin-like substance".

Syrian President Bashar al-Assad claimed to AFP the reports were "100 percent for us a fabrication" to justify a US military strike on a Syrian air base.

The fact-finding mission is hoping to complete its work in the next two to three weeks.

"Our experts are fully aware of the significance of the task they are expected to fulfil and I am confident that they will do it in a professional and impartial manner using all available technical means," said Uzumcu.