Russia says it will not shoot down US missiles if Donald Trump orders another Syrian airstrike because they are 'there to fight terrorism, not external threats'.

America unleashed a wave of 59 Tomahawk strikes on one of Bashar al-Assad's airbases in the wake of a horrifying chemical attack on 87 Syrian civilians last week.

Russia - which has backed the Syrian dictator military - as well as Iran and its allies said the US had crossed a 'red line' and warned they 'will respond to any aggression' in the future.

But today the head of the Russian Federation Council's Defense and Security Committee insisted Kremlin forces stationed in Syria will not try to intercept US missiles should Washington order further strikes.

Russia says it will not shoot down US missiles if Donald Trump orders another Syrian airstrike because they are 'there to fight terrorism, not external threats'. America unleashed a wave of 59 Tomahawk strikes (pictured) on one of Bashar al-Assad's airbases in the wake of a horrifying chemical attack on 87 Syrian civilians last week

Russia - which has backed the Syrian dictator military - as well as Iran and its allies said the US had crossed a 'red line' and warned they 'will respond to any aggression' in the future. Russian President Vladimir Putin is pictured

Viktor Ozerov told the news agency Interfax: 'Our armed forces are in Syria to fight terrorism — not to defend against external threats. That’s not our mandate, and we’re not going to intercept anything.'

But he also warned that the Syrian military still had a 'legal right' to try to shoot down rockets.

Some 87 people, including children, are believed to have been killed in a suspected sarin nerve agent strike on the rebel-held town of Khan Sheikhoun.

US president Donald Trump ordered a strike by 59 cruise missiles on the base in America's first direct attack against the Syrian government.

Last night, a senior US official said America had concluded Russia knew in advance of Syria's chemical weapons attack.

The official said a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment.

Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

Some 87 people, including children, are believed to have been killed in a suspected sarin nerve agent strike on the rebel-held town of Khan Sheikhoun

A devastated father was pictured cradling the bodies of his dead twins after they were killed during the chemical attack in Khan Sheikhoun, in the rebel-held central province of Idlib, Syria

The official said the presence of the surveillance drone over the hospital couldn't have been a coincidence, and that Russia must have known the chemical weapons attack was coming and that victims were seeking treatment.

The official, who wasn't authorised to speak publicly on intelligence matters and demanded anonymity, didn't give precise timing for when the drone was above the northern Syrian town.

He also didn't provide all the details for the military and intelligence information that form the basis of what he said the Pentagon has now concluded.

The allegation is grave, even by the standards of the currently dismal U.S.-Russian relations.

Russian President Vladimir Putin won't meet with US Secretary of State Rex Tillerson (centre) when he's in Moscow on Wednesday - an apparent snub after the Trump administration dropped bombs on Syria last week

Although Russia has steadfastly supported Syrian President Bashar Assad's government, and they've coordinated military attacks together, Washington has never previously accused Moscow of complicity in any attack that involved the gassing of innocent civilians, including children.

The former Cold War foes even worked together in 2013 to remove and destroy more than 1,300 tons of Syrian chemical weapons and agents.

Meanwhile, Russia's president, Vladimir Putin, is due to snub Secretary of State Rex Tillerson when he's in Moscow on Wednesday. Putin is passing Tillerson off to his foreign minister, Sergey Lavrov.

The Kremlin did not say why the Russian president was avoiding Tillerson after meeting repeatedly with his predecessor, John Kerry, in the Obama years.

But it had ripped the US for its air assault in Syria and warned Donald Trump that he was 'one step from war' with the country after the attack.

Levelling the charge that Putin's military knew about the attack in advance will only increase the tension.