Russia's ambassador to Lebanon has said any US missiles fired at Syria would be shot down and the launch sites targeted, a step which could trigger a major escalation in the Syrian war.

Alexander Zasypkin, in comments broadcast on Tuesday evening, said he was referring to a statement by Russian president Vladimir Putin and the Russian chief-of-staff.

"If there is a strike by the Americans then... the missiles will be downed and even the sources from which the missiles were fired," he told Hezbollah's al-Manar TV.

He also said a clash "should be ruled out and therefore we are ready to hold negotiations".

On Tuesday, Russia and the US blocked attempts by each other in the UN Security Council to set up international investigations into suspected chemical weapons attacks in Syria.

Each country placed a resolution before the Security Council to vote on. The US text was vetoed by Russia, while the Russian text was not adopted because it did not gain enough votes. 

The deadlock came as Syria invited a mission from the international chemical weapons watchdog to investigate the suspected poison gas attack near Damascus over the weekend.

Its 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. 

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

The White House announced on Tuesday that 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”. 

The US president promised to respond "forcefully" to the alleged chemical attack, and that the US had "a lot of options militarily". 

Theresa May joined Mr Trump in calling for a response to the Syrian regime's latest use of chemical weapons, "if confirmed". 

In separate phone calls, Ms May, Mr Trump and French president Emmanuel Macron agreed the international community should work together to hold Mr Assad's government and its backers to account.