(REUTERS) - International officials are working to try to confirm a suspected poison gas attack in the besieged rebel-held Syrian town of Douma on Saturday (April 7), which aid groups said killed dozens of people.

US President Donald Trump warned there would be a "big price to pay" for the attack. The Syrian state denied government forces had launched any chemical assault.

Following is a timeline of investigations into Syria's chemical weapons.

2013

- US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov reach an agreement on the elimination of Syria's chemical weapons.

- UN Secretary General Ban Ki-moon establishes United Nations Mission to Investigate Allegations of the Use of Chemical Weapons in the Syrian Arab Republic. Headed by Swedish scientist Ake Sellstrom and comprised of experts from the World Health Organization and the Organisation for the Prohibition of Chemical Weapons (OPCW), it is tasked with looking into possible use of chemical weapons following reports of an attack in the northern town of Khan al-Assal.

It confirms use of sarin in the Aug 21 attack in the Damascus suburb Ghouta that killed hundreds.

2014

- OPCW Declaration Assessment Team begins work to resolve gaps and inconsistencies in Syria's declaration to the OPCW, which was supposed to include all aspects of its chemical weapons programme.

By April 2018, after visiting Syria at least 18 times to inspect sites and meet Syrian officials, the technical secretariat of the OPCW remained unable to "verify that Syria had submitted a declaration that could be considered accurate and complete."

- OPCW establishes a fact-finding mission in response to persistent allegations of chemical weapon attacks in Syria, "to establish facts surrounding allegations of the use of toxic chemicals, reportedly chlorine, for hostile purposes in the Syrian Arab Republic."

The mission concludes the use of chlorine was systematic and widespread. It is not tasked with assigning blame.

2015

- United Nations-OPCW Joint Investigative Mechanism in Syria is established to identify individuals or entities behind chemical weapons attacks.

2016

- UN-OPCW Joint Investigative Mechanism concludes that Syrian government forces used chlorine as a chemical weapon in three cases and that Islamic State in Iraq and Syria militants used sulphur mustard.

- OPCW executive council adopts decision condemning Syrian government and Islamic State for chemical weapons use after a vote that split the body and signalled an end to US-Russia cooperation.

2017

- OPCW fact-finding mission concludes that sarin was used in an April 4 attack in the Khan Sheikhoun area of northern Syria, the most deadly use of the nerve agent in three years. It does not assign blame.

- The Syrian government of President Bashar al-Assad is blamed by the Joint Investigative Mechanism for the sarin attack on opposition-held Khan Sheikhoun that killed dozens of people.

- Russia casts a veto at the UN Security Council preventing the renewal of the mandate for the Joint Investigative Mechanism. The investigation ends in November 2017 after Russia repeatedly blocks attempts at renewal.

2018

- Diplomats and scientists tell Reuters that the Syrian government's chemical weapons stockpile has been linked for the first time by lab tests to the largest sarin nerve agent attack of the civil war.

Laboratories working for the OPCW compared samples taken by a UN mission in the Damascus suburb of Ghouta after the Aug 21, 2013 attack to chemicals handed over by Damascus for destruction in 2014.

The tests found "markers" in samples taken at Ghouta and at the sites of two other nerve agent attacks, in Khan Sheikhoun in April 2017 and Khan al-Assal in March 2013, two people involved in the process say.