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

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

The Russian military said on March 13 that it would respond to any U.S. strike on Syria, targeting any missiles and launchers involved in such an attack. Russia is Syrian President Bashar al-Assad's most powerful ally.

The United States and its allies are considering whether to hit Syria over a suspected poison gas attack that medical relief organisations say killed dozens of people in the rebel-held town of Douma near Damascus on Saturday.

"If there is a strike by the Americans, then...the missiles will be downed and even the sources from which the missiles were fired," Zasypkin told Hezbollah's al-Manar TV, speaking in Arabic. He also said a clash "should be ruled out and therefore we are ready to hold negotiations."

Russia and the United States blocked attempts by each other in the UN Security Council on Tuesday to set up international investigations into chemical weapons attacks in Syria.

U.S. President Donald Trump on Tuesday cancelled a planned trip to Latin America later this week to focus instead on responding to the Syria incident, the White House said. Trump had on Monday warned of a quick, forceful response once responsibility for the Syria attack was established.

Meanwhile, pan-European air traffic control agency Eurocontrol is warning airlines to exercise caution in the eastern Mediterranean due to the possible launch of airstrikes into Syria in next 72 hours.

Eurocontrol said that air-to-ground and cruise missiles could be used within that period and there was a possibility of intermittent disruption of radio navigation equipment.

The Eurocontrol warning on its website did not specify the origin of any potential missile threat.