A senior US official has said that US investigations have concluded that Russia was aware in advance of the Syrian government’s alleged chemical weapons attack on a rebel-held village.

A Russian-operated drone flew over a hospital building in Idlib province on April 4 as victims were rushed there for treatment, a Trump administration official said. The hospital was then bombed by either Russian or Syrian forces in what American intel believes was an attempt to cover up the alleged usage of chemical weapons.

The drone’s presence - which before Monday the US had not known was Russian-operated - “could not have been a coincidence,” an official speaking on the condition of anonymity told the Associated Press.

Spicer slips up in press briefing and says America's aim is to destabilize Syria

The official did not give precise timing for when the drone was in the area, where more than 80 people were killed by what the Turkish Health Ministry said autopsies showed was exposure to both sarin and chlorine gas.

The official also didn't provide details for the military and intelligence information that form the basis of what the Pentagon now believes.

The US has no proof of Moscow's involvement in the alleged initial attack, another senior official cautioned, after the grave allegation - the first direct assertion from the US Moscow could be complicit in attacking civilians - was put to them.

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

Both Damascus and Moscow have denied the regime used chemical weapons in Khan Sheikhoun, maintaining that the casualties were caused by gases released after an al-Qaeda-affiliated ammunitions depot was hit by conventional munitions in a government air raid.

Neither government has produced any evidence to back up the claim, which has been derided by medical NGOs and Western intelligence services.

The US decision to launch a retaliatory strike against a Syrian regime airbase last Friday has further complicated the already dazzlingly complex Syrian civil war. President Donald Trump’s administration had up until last week prioritised defeating Isis as the US’ main focus in the six-year-old conflict, rather than removing Syrian President Bashar al-Assad from power.

On Monday UK Foreign Secretary Boris Johnson said that Russian parties could possibly be subject to economic sanctions over the country’s support for Mr Assad.

Prime Minister Theresa May and Mr Trump spoke on the phone ahead of the G7 meeting in Italy on Tuesday, agreeing there is now a “window of opportunity” to persuade Russia to ditch its support for the Syrian regime, officials said.