International inspectors have entered the Syrian town where an alleged chemical attack was carried out earlier this month, Syrian state media reports, after they had waited for days in the capital for permission from Syrian and Russian authorities.

The fact-finding mission from the Organisation for the Prohibition of Chemical Weapons (OPCW) is investigating reports that government forces launched an April 7 chemical attack in the final stages of their fight to retake the town of Douma from rebels.

The alleged gas attack, which Syrian activists say killed more than 40 people, prompted punitive US, British and French airstrikes.

Syria and its ally Russia deny any chemical attack took place, and Russian officials have accused Britain of staging a “fake” chemical attack.

British Prime Minister Theresa May says Syria and Russia – whose forces now control the town east of Damascus – are trying to cover up evidence.

Journalists were allowed access to the suspected attack sites on Monday, but the OPCW said Syrian and Russian authorities blocked the inspectors.

The Associated Press spoke to survivors and witnesses who described being hit by gas.

Several said a strange smell started spreading and people screamed, “It’s chlorine! It’s chlorine!”

The US and France say they have evidence that Syrian President Bashar Assad’s forces used poison gas in the attack, but they have not provided it, even after Saturday’s strikes.

Russian military said on Tuesday it had found a laboratory in Douma.

The lab contained chlorine and various ingredients for mustard gas, Russian chemical weapons specialist Alexander Rodionov said in comments carried by state news agency TASS.

“It is possible to conclude that this laboratory was used by illegal armed groups to produce poisonous substances,” Mr Rodionov said.

Douma was the last rebel-held town near Damascus, and the target of a government offensive in February and March that killed hundreds of people and displaced tens of thousands.

Hours after the alleged chemical attack, the rebel faction that controlled the town, the Army of Islam, relented and was evacuated along with thousands of residents.