The exercise is part of an ongoing investigation into the suspected chemical attack by the Syrian regime that killed dozens in the enclave on April 7. Meanwhile, the world donors have pledged $4.4 billion for people affected by Syrian war.
Inspectors with the global chemical weapons agency on Wednesday visited a second site in Syria's Douma and took samples to help them determine whether banned toxic munitions were used there.
The Organisation for the Prohibition of Chemical Weapons is investigating the deaths of dozens of people in the enclave outside of the Syrian capital on April 7.
The attack led to air strikes by the United States, France and Britain against sites in Syria. They accused the regime leader Bashar al Assad of using chemical weapons, possibly a nerve agent.
Syria and its ally Russia have denied the accusation and said rebel forces staged the attacks.
The fact-finding mission (FFM) arrived in Damascus on April 14, but was delayed by a week before it could get to sites it deemed of interest.
Meanwhile, international donors on Wednesday pledged $4.4 billion in aid for war-hit civilians of Syria.
Syria accused of delaying tactics
Western powers accused Russia and Syria of stalling tactics to delay the process and of tampering with evidence that may have pointed to government involvement.
Samples taken from two sites will be returned to the OPCW's laboratory in the Netherlands and then sent on to affiliated laboratories for examination, the OPCW said in a statement.
Those tests will determine whether or not chemical weapons were used.
The OPCW does not assign blame for chemical attacks and there is currently no mechanism that can do so after Russia vetoed a resolution to extend the mandate of a United Nations-OPCW mission in November.
Russia to hold briefing at OPCW
The Russian delegation to the OPCW will hold a briefing on Thursday in The Hague at which it will present "some Syrians to speak about the reported Douma incident," the agency's statement said.
It was unclear who Russia planned to present, but the OPCW said they have not been interviewed by investigators.
"The FFM will continue to carry out its independent and impartial mission based on interviews with relevant people, its findings from the site visits, analysis of the sample results, as well as any other information and materials collected," it said.
Fresh air strikes
Meanwhile, Syrian regime carried out air strikes in Homs, Idlib and Hama provinces on Wednesday, according to White Helmets.
A video published by the emergency and rescue service showed pictures of damaged homes after four air raids in al Rastan city, north of Homs in central Syria.
The strikes reportedly left unknown number of civilians injured.
Donors pledge aid
International donors on Wednesday pledged $4.4 billion in aid for civilians caught up in the Syrian civil war – well short of what the UN says is needed for humanitarian work in Syria and neighbouring countries.
The sum committed at a two-day conference in Brussels was less than half of the $9 billion the UN says is needed this year to help those in need inside Syria and living as refugees in neighbouring countries.
The head of the UN aid agency UNOCHA called the $4.4 billion "a good start," but a group of nine international aid organisations said the conference "did not go nearly far enough."
"My best guess is that by the end of the day we will have heard pledges for 2018 of $4.4 billion," Mark Lowcock, the head of UNOCHA, told a news conference.
He added that pledges of a further $3.3 billion for 2019 and after were expected at the conference, attended by more than 80 countries, aid groups and agencies.
Britain announced $630 million for 2018 and another $418 million for 2019, while Germany said it would donate $1.218 billion and the EU pledged some $791.8 million.
But several major donors including the US have not yet confirmed their pledges, Lowcock said, because of ongoing internal budget wrangling.
More than 700,000 people have been displaced since the start of this year alone as Assad has stepped up his offensive against rebel forces, intensifying the humanitarian crisis.
Unacceptable.— UNICEF (@UNICEF) April 25, 2018
Children in #Syria account for one quarter of civilian deaths.
We’re calling on all parties to STOP attacks on schools, hospitals and homes.#ChildrenUnderAttack #NotATarget #SyriaConf2018 pic.twitter.com/ROqFTrHYXF
Struggling peace talks
Europe hoped to use the conference to reinvigorate the faltering UN-led peace process in Geneva, but it was not clear how effective the push was.
EU diplomatic chief Federica Mogherini urged Moscow and Tehran, Assad's key supporters, to help bring him to the negotiating table, saying they had a duty to help wind down the war, now in its eighth year.
"We need in particular Russia, Iran to exercise pressure on Damascus so that it accepts to sit at the table under UN auspices," Mogherini said as she arrived for the gathering, the seventh of its kind.
"We believe that the only sustainable peace for Syria will be linked to a political process under UN auspices."