BEIRUT/MOSCOW • President Bashar al-Assad's government has invited international inspectors to send a team to Syria to investigate an alleged chemical attack in the town of Douma, in an apparent move to avert possible Western military action over the incident.

At least 60 people were killed and more than 1,000 injured in last Saturday's suspected attack on Douma, then still occupied by rebel forces, according to a Syrian relief group.

United States President Donald Trump on Monday warned of a quick, forceful response once responsibility was established, although he appeared to have little doubt it was the work of Mr Assad's Russian-backed forces. The Syrian government and Russia said there was no evidence that a gas attack had taken place, and that the claim was bogus.

The incident has thrust Syria's seven-year-old conflict back to the forefront of international concern. Mr Trump will miss a Latin American summit in Peru this week in order to focus on the crisis, the White House said.

Adding to the volatile situation, Iran, Mr Assad's main ally along with Russia, threatened to respond to an air strike on a Syrian military base on Monday that Teheran, Damascus and Moscow have blamed on Israel.

Meanwhile on the ground, thousands of militants and their families arrived in rebel-held north-western Syria after surrendering Douma to government forces. The evacuation deal restores Mr Assad's control over the entire eastern Ghouta - formerly the biggest rebel bastion near Damascus.

The Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) is already at work trying to establish what exactly took place in Douma.

But whether a team would try to get there was unclear. OPCW inspectors have been attacked on two previous missions to the sites of chemical attacks in Syria.

"Syria is keen on cooperating with the OPCW to uncover the truth behind the allegations that some Western sides have been advertising to justify their aggressive intentions," state news agency Sana said, quoting an official Foreign Ministry source.

On Monday, Mr Trump told a meeting of military leaders and national security advisers in Washington that he would take a decision that night or shortly after on a response, and that the US had "a lot of options militarily" on Syria.

"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," Mr Trump said.

At the United Nations Security Council, the US planned to call for a vote yesterday for a new inquiry into responsibility for the use of chemical weapons in Syria, diplomats said. If the US proposal is put to a vote, it is likely to be vetoed by Russia.

At a meeting on Monday, US Ambassador to the UN Nikki Haley said Washington would respond to the suspected attack in Syria, whether the Security Council acted or not.

"This is basically a diplomatic set-up," said Mr Richard Gowan, a UN expert at the European Council on Foreign Relations.

"Russia will inevitably veto the US resolution criticising Assad, and Washington will use this to justify military strikes," he said. "A breakdown at the UN will also make it easier for France to justify strikes."

France said yesterday it would respond if it was proven that Mr Assad's forces carried out the attack.

US officials said that Washington was weighing a multinational military response.

Russian UN Ambassador Vassily Nebenzia accused the US, France and Britain of stoking international tensions by engaging in a "confrontational policy against Russia and Syria".

"Russia is being unpardonably threatened. The tone with which this is being done has gone beyond the threshold of what is acceptable, even during the Cold War."

REUTERS