International chemical weapons inspectors have collected samples of soil and human tissue from the Syrian town of Douma in an effort to establish whether such weapons were used in an attack there.

The Organisation for the Prohibition of Chemical Weapons (OPCW) said it would now consider whether the team needs to make a second visit to the town where at least 40 people are thought to have died from the suspected gas attack.

The samples will be transported to the Netherlands and then on to the organisation’s designated labs for analysis.

The experts there will try to determine whether chemical weapons were used in the attack on 7 April, and if so, which.

The US, UK and France carried out joint missile strikes, as a result of the suspected attack, on sites that they said were linked to Syrian chemical weapons programmes.

The OPCW said that based on analysis of the sample results, as well other information and materials collected by the team, the mission would compile a report and submit it to the organisation’s member states.

The organisation is not mandated to conclude which side in the conflict used chemical weapons.

Inspectors had arrived in Damascus last weekend to examine the site of the suspected attack, but the team faced several days of delays after an advance security detail was fired upon on Tuesday.

A Russian foreign ministry spokesperson said that Moscow expected the inspectors to carry out an “impartial investigation into all the circumstances of what has occurred”, Russia’s Interfax news agency reported.

The French foreign minister, Jean-Yves Le Drian, said on Friday that Russia was obstructing the inspectors’ access to the town.

He said it was likely that the obstruction was aimed at ensuring that proof of the attack disappeared.

But Russia’s foreign ministry spokeswoman, Maria Zakharova, condemned the delay as “unacceptable”.

Moscow, whose forces in Syria back President Bashar al-Assad’s army, has denied that Syrian forces carried out the alleged attack.

The OPCW has been investigating use of toxic chemicals in Syria’s civil war since 2014.

Meanwhile, Syrian rebels have started to evacuate three towns in the eastern Qalamoun region in the Damascus countryside, according to Syrian state media.

The evacuations are the latest in a string of population transfers around the Syrian capital that have displaced more than 60,000 people as the government reconsolidates its control.