Syrian President Bashar al-Assad had moved most of his combat aircraft to Russian airbases in an attempt to protect them from a further US attack, say reports.

The move follows the crippling US assault on the Shayrat airbase two weeks ago which destroyed around 20% of the regime's operational warplanes, according to US Defence Secretary Jim Mattis.

The US cruise missile strike was in retaliation for a suspected sarin attack that was unleashed on the rebel-held town of Khan Sheikhoun on Tuesday (4 April) that killed 89 people.

The Syrian government has repeatedly denied involvement in the alleged dropping of nerve gas on the rebel-held town in Idlib.

However, the US administration has affirmed that the 59 Tomahawks fired at the base was the correct response to the alleged chemical attack, and has reiterated that they may do the same again in the face of a further chemical attack.

Fearing such further strikes, the Syrian regime have now moved many of their working combat aircraft to the Hmeimim airbase and the nearby Bassel al-Assad international airport in Latakia, reports The Times.

The base, in a coastal Assad stronghold, is one of many used by Russians for their operations in the country targeting Isis and rebels, a US official told CNN.

Khan Sheikhoun crater
An image from the suspected crater of a chemical weapons attack in Khan Sheikhoun, Syria. Social media

It has been reported that the base is protected by Russia's S-400 air-defence system, capable of long-range air-defence at a radius of 150 miles.

Bombing the base would also risk damaging the 30 or so fixed-wing aircraft and 15 helicopter gunships Russia has deployed at the airbase, or killing some of the hundreds Russian troops reportedly based there.

Mattis said that 20% of Assad's 100 to 120 operational aircraft were destroyed on 6 April, with the Kremlin labelling the US strike an "action of aggression" against a sovereign state.

Syria strike
Battle damage assessment image of Shayrat Airfield, Syria, is seen in this Digital Globe satellite image, released by the Pentagon following US Tomahawk Land Attack Missile strikes from Arleigh Burke-class guided-missile destroyers, the USS Ross and USS Porter Reuters

Assad claimed the toxic gas attack had been faked by the rebels and the West to justify the Shayrat airbase strike. The embattled dictator even questioned whether the dead children shown in several videos were "dead at all".

Despite Assad's claims, arms watchdog the Organisation of the Prohibition of Chemical Weapons say samples taken from ten victims analysed at four laboratories showed "incontrovertible" results that a sarin-like substance was present.

The World Health Organisation and aid group Médicines Sans Frontières have also separately issued statements saying symptoms suffered by victims suggested a neurotoxic agent was used.