A senior United States official speaking to the Associated Press on condition of anonymity has said that the US has concluded that Russia knew ahead of time about the chemical attack that took place in Syria last week.

In what officials said may have been an attempt to cover-up the use of chemical weapons, a Russian-made jet bombed a hospital where victims of the attack in Khan Sheikoun were seeking treatment. Hours before, a drone had flown over the hospital - though it was unknown at the time whether the drone was operated by Russian or Syrian authorities.

The senior official told the AP that the drone was Russian-operated and that they must have known in advance that the attack would happen and that the victims would seek treatment.

The chemical attack in Khan Sheikhoun allegedly purportrated by the Russia-backed Assad regime was the impetus behind Trump firing missiles at an airbase from which the jets that attacked the town are believed to have taken off.

The accusation is likely to make Secretary of State Rex Tillerson's upcoming visit to Moscow even more tumultuous. After Trump ordered the strike on Thursday (6 April), Tillerson said that Russia was either "complicit" or "simply incompetent" on Assad's chemical weapons.

If Russia is shown to have been complicit, it would signal an end to an Obama-brokered agreement between the superpowers to stop Assad using chemical weapons.

Upon ordering the strike, Trump said that it was "in the vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons." Adding, "There can be no dispute that Syria used banned chemical weapons, violated its obligations under the chemical weapons convention, and ignored the urging of the UN security council."

UK Foreign Secretary Boris Johnson cancelled a planned visit to Moscow after the chemical weapons attack, saying his priority was "to continue contact with the US and others in the run up to the G7 meeting."

Russia said that the strike on Khan Sheikhoun had been undertaken to destroy a chemical weapons munitions depot controlled by rebel forces.

Johnson replied to the statement by saying that "all the evidence I've seen suggests that this was the Assad regime... using illegal weapons on their own people."