The global body supervising the surrender of Syria's chemical weapons is to investigate fresh claims that a less dangerous – but still lethal – chlorine gas has been used in recent attacks on opposition areas.

The Organisation for the Prohibition of Chemical Weapons (OPCW) has announced it will investigate allegations that chlorine has been used at least nine times since February, killing scores of people and wounding hundreds more.

The move follows intensive lobbying from the US and France who have both indicated in the past fortnight that they believe the Syrian government has been responsible for the attacks.

Activists have chronicled the aftermath of all the incidents and, in some cases, shot video of helicopters dropping large explosive barrels that emitted noxious clouds across areas in which residents showed symptoms of being exposed to gas.

OCPW head Ahmet Uzumcu said in a brief statement that a "mission would be created to establish facts" surrounding the latest allegations.

The new mission comes as the complicated task of withdrawing Syria's 1,300-tonne supply of sarin, mustard gas, and the precursor chemicals used to mix and weaponise them, nears completion. The OPCW announced earlier this week that 92.5% of the chemicals – the core of the Syrian military's stockpile – had so far been moved to the port of Tartous in the country's north-west.

From there they will be transferred to Danish and Norwegian ships that will transport them to Spain, where they will be moved to another ship moored waiting for them at a US Navy base in Cadiz. While the surrender process has run behind schedule, it has taken place without disruption or sabotage.

Sarin was used in a mass attack on opposition-held areas near Damascus last August, killing between 355 and 1,400 people and prompting the threat of a US strike, which was averted by a deal brokered by Russia for Syria to hand over its stockpiles.

At the time Syrian officials said all its supplies would be surrendered. However, western officials have suggested that some chemicals used by the military, such as chlorine, may not have been declared.

Claims that chlorine, or some sort of industrial chemical, had been used in new strikes first surfaced in February after an attack near Damascus.

Other incidents are alleged to have taken place in Idlib and Homs province. Clothing and blood samples of those affected by gas have been smuggled out of Syria by opposition groups and activists for testing in laboratories in Turkey and France.

Washington said last week it suspected the Syrian regime had been responsible for the incidents, that are thought to have killed between 20-30 people.

The OPCW move came as the former head of the UK Defence Ministry's Chemical Biological Radiological and Nuclear regiment claimed to have conclusive proof that Syrian forces were responsible for the attacks, which had used chlorine and ammonia.

"We have unequivocally proved that the regime has used chlorine and ammonia against its own civilians in the last two to three weeks," Hamish de Bretton-Gordon said after receiving three soil samples provided by the Daily Telegraph. He said he could definitively establish chain of custody of the samples, which were taken from areas hit by gas bombs in Idlib province.

The Foreign and Commonwealth Office (FCO) welcomed the new investigation. "We hope the OPCW mission will establish the facts," it said in a statement.

"While we believe that some groups operating in Syria are interested in developing a chemical capability, and some have acquired critical components, there is no evidence that any other than the regime possess a chemical weapons capability at present. The regime, by contrast, used chemical weapons in 2013 and has now missed the deadlines set out for the removal of chemicals from Syria."

The question of who used chemical weapons has been the most bitterly contested of a long list of atrocities in Syria's three year war, which has seen more than 150,000 people killed, around nine million people exiled and large parts of the country destroyed.

Syrian officials have escalated their use of force as the conflict has worn on, sending ballistic missiles into rebel-held areas from January last year and now blitzing opposition parts of Aleppo with barrel bombs that are laying waste to all below and emptying neighbourhoods of remaining residents.

Human Rights Watch said on Tuesday that satellite imagery had identified at least 85 major impact sites in eastern Aleppo between 22 February and 2 April alone. Opposition activists say attacks have intensified since and were also heavy in January.

Opposition forces, among them global jihadists, have also targeted civilian areas. A car bomb in a loyalist area of Homs killed up to 36 people on Tuesday, state media reported. A separate mortar attack in Damascus killed another 14 people, all civilians.