Syria says it has invited a mission from the international chemical weapons watchdog to investigate a suspected poison gas attack near Damascus.

The country’s foreign ministry said it would help the Organisation for the Prohibition of Chemical Weapons (OPCW) in a fact-finding mission into the alleged attack, which opposition activists say killed 40 people over the weekend.

The Syrian regime and its Russian backers deny it used chemical weapons in Douma, the last rebel-held town in the eastern Ghouta suburbs.

Earlier on Tuesday, a senior Russian lawmaker said Moscow was willing to help the OPCW visit Douma, so it could ”finally start carrying out the functions it was created for”.

It comes as the White House announced Donald Trump would skip an upcoming summit in South America and instead remain in the US to “oversee the American response to Syria and to monitor developments around the world”.

On Monday, the president said he would respond “forcefully” to the alleged chemical attack, and that the US has “a lot of options militarily”.

“But we can’t let atrocities like we all witnessed... we can’t let that happen in our world... especially when we’re able to because of the power of the United States, the power of our country, we’re able to stop it,” he said.

Eastern Ghouta bombings: Syrian war in pictures Show all 14 1 /14 Eastern Ghouta bombings: Syrian war in pictures Eastern Ghouta bombings: Syrian war in pictures A Syrian woman and children run for cover amid the rubble of buildings. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Smoke rises from buildings following the attack on the village of Mesraba in the rebel-held besieged Eastern Ghouta region on the outskirts of the capital Damascu. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Injured children receive medical treatment. EPA Eastern Ghouta bombings: Syrian war in pictures A Syrian man carries a child injured. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures An injured child receives treatment following bombings on several areas of eastern Ghouta. EPA Eastern Ghouta bombings: Syrian war in pictures A child reacts inside a hospital after relatives were injured in the bombing. EPA Eastern Ghouta bombings: Syrian war in pictures Syrian children cry at a make-shift hospital in Douma following air strikes on the Syrian village of Mesraba. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Syrian Civil Defense group extinguishing a store during airstrikes and shelling by Syrian government forces. AP Eastern Ghouta bombings: Syrian war in pictures A wounded 12-year-old Syrian boy, cries as he receives treatment at a make-shift hospital. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Syrians carry a wounded man. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures An injured man covered with blood at a medical point. Reuters Eastern Ghouta bombings: Syrian war in pictures People sit a medical point in the besieged town of Douma, Eastern Ghouta. Reuters Eastern Ghouta bombings: Syrian war in pictures Syrian Civil Defense running to help survivors. AP Eastern Ghouta bombings: Syrian war in pictures Injured children receive medical treatment. EPA

The British government said on Tuesday military intervention in Syria was "a live issue" and that it was discussing options with its allies.

Speaking during a tour of British-funded aid projects in Jordan, Penny Mordaunt, the international development secretary, branded the alleged poison gas attack an "atrocity".

The threat of a US strike, which Russia warned could lead to “grave repercussions”, has prompted Bashar al-Assad’s forces to take precautionary measures at military bases and posts in government-held areas, according to war monitors.

Dozens of buses arrived in rebel-held Douma on Tuesday, carrying Syrian insurgents who agreed to surrender following the suspected chemical attack, a monitoring group said.

Their evacuation, along with their families, came as the United Nations Security Council prepared to meet to discuss the crisis.

The pro-Assad Watan newspaper said about 40,000 militants and their families were due to leave in all, as the UN refugee agency voiced alarm at “spiralling new displacement”.

Tony Blair: Nonintervention in Syria has consequences

The deal restores Mr Assad’s control over the whole of eastern Ghouta – formerly the biggest rebel bastion near Damascus.

Dozens of buses carrying hundreds of fighters, along with family members and civilians who did not wish to come back under Assad’s rule, reached opposition areas near Aleppo on Tuesday, the Syrian Observatory for Human Rights said.

As part of the surrender deal, the Jaish al-Islam group that controlled the town released scores of people it had been holding.

The deal took effect on Sunday, hours after medical aid groups reported the suspected chemical attack, saying it had killed dozens of people in Douma.

Adding to the volatile situation, Iran, Mr Assad’s closest ally alongside Russia, said an air strike on a Syrian military base that Tehran and Moscow blames on Israel “will not remain unanswered”.

Monday’s pre-dawn strike on the air base in central Homs reportedly killed 14 people, including seven Iranians.