A global chemical weapons watchdog says it has deployed fact-finding teams to investigate a suspected chemical attack that hit Douma, a town in Syria’s Eastern Ghouta and the last rebel stronghold.

The announcement came on Thursday, just days after the suspected attack which, according to activists and medics on the ground, claimed the lives of more than 85 civilians and injured at least 1,200 people.

The victims of the April 7 incident were mostly women and children, activists told Al Jazeera from Eastern Ghouta, a Damascus suburb.

"The Prohibition of Chemical Weapons (OPCW) confirms that the OPCW Fact-Finding Mission (FFM) team is on its way to Syria and will start its work as of Saturday 14 April, 2018,” the watchdog said in a statement on Thursday.

Later on Thursday, French President Emmanuel Macron said his country had proof that the Syrian government launched the gas attacks.

Earlier in the day, Bashar al-Jaafari, Syrian ambassador to the US, said in New York that two investigating teams from the OPCW were scheduled to arrive in Syria within the next 24 hours.

Al-Jaafari was quoted by SANA news agency as saying that the Syrian government was ready to facilitate the teams’ entry into any point in Douma, Eastern Ghouta’s largest town.

Meanwhile, James Mattis, the Pentagon chief, has called for an American investigation into the suspected attack.

Speaking to Congress at a hearing of the House Armed Services Committee on Thursday, Mattis said he believed a chemical attack had taken place and blamed Russia for being complicit in Syrian government’s possession of chemical weapons.

"I believe there was a chemical attack and we are looking for the actual evidence,” he said, indicating that a decision is yet to be made.

The remarks came a day after US President Donald Trump warned Russia that his country would launch missile attacks on Syria after the alleged chemical attack took place.

In response, Maria Zakharova, Russia’s foreign ministry spokesperson, said allegations of the use of chemical weapons in Douma were false and could not be used as a pretext to undertake any military action against Syria.

Peskov also expressed Moscow’s support for "serious initiatives” on the Syrian crisis and other international issues.

Russia has been involved in the Syrian conflict since 2015 and became Syrian President Bashar al-Assad’s main ally in the war against armed opposition groups.

With Russian military assistance, Assad launched a military offensive on Eastern Ghouta, which had been under rebel control since mid-2013.

Since the start of the aerial bombardment campaign, at least 1,600 civilians have been killed and more than 130,000 people displaced, mostly to other areas in the country, according to the UN.