A stockpile of sarin was being held at the Syrian airbase targeted by US air strikes following a chemical attack, American officials have said.

Dozens of cruise missiles were fired from two American warships stationed in the Mediterranean Sea in the early hours of Friday morning, days after more than 70 people died in a rebel-held town.

They hit Shayrat airbase in the province of Homs, killing at least six of Bashar al-Assad’s troops and destroying planes, ammunition stores and buildings.

Donald Trump’s national security adviser, Herbert “HR” McMaster, told a press conference the attack aimed to reduce the airfield’s ability to “continue mass murder attacks against Syrian civilians”.

“There were measures put in place to avoid hitting what we believe is a storage of sarin gas, so that that would not be ignited and cause a hazard to civilians or anyone else,” he said.

Rex Tillerson, the US Secretary of State, said the administration had a “very high level of confidence” that the massacre in Khan Sheikhoun was carried out by Assad’s forces using sarin, which is banned as a weapon of mass destruction under international law.

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

Doctors said victims started to choke, vomit and convulse with foam coming out of their mouths and pin-point pupils, showing symptoms of exposure to the nerve agent, with analysts from international charities supporting the claims.

Assad’s government claims it destroyed its stockpiles of sarin following an international agreement struck in 2013, when the nerve agent was used to kill hundreds of civilians in Ghouta and almost sparked international intervention against the Syrian regime.

Its ally Russia also claims Syria no longer possesses or uses chemical weapons, and claimed the toxic gas that killed civilians in Khan Sheikhoun emanated from a rebel warehouse that was struck by legitimate air strikes.

Rebels denied the Russian defence ministry’s claims, while experts told The Independent they were not credible.

Beyza Unal, a research fellow with the International Security Department at Chatham House, said sarin is expensive and difficult to purify and store.

“Something that needs a certain level of expertise and also money,” she told The Independent, saying any facility would need the ability to take oxygen out of the area where sarin is stored.

“I don’t think rebel groups would have the ability governments would have to purify nerve agents to a level that would make them stable,” Dr Unal added.

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)

“I don’t buy the Russian claims…the story doesn’t add up.”

She added that images of bomb craters in Khan Sheikhoun indicated small payloads, rather than explosives of the type typically used to destroy an entire building.

American officials said the attack was traced to Shayrat airbase, with Mr Tillerson describing it as “the facility that delivered” the bombardment.

The Syrian army said at least six soldiers were killed, with state media reporting the additional deaths of at least seven civilians, including four children.

Russia was among the countries that also used the base, but the US warned Vladimir Putin’s forces ahead of the strike and none of the Kremlin’s troops were killed.

A spokesman for Vladimir Putin said the strike had seriously damaged ties between Washington and Moscow, which has pledged to increase the Syrian government’s air defences and suspended an agreement aiming to avoid conflict with US forces.

Mr Trump announced he had ordered the attack, calling Assad a “dictator” who had “launched a horrible chemical weapons attack on innocent civilians”.

Shayrat airfield in Syria (Getty Images)

“Tonight I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched,” the President added.

“It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.”

Critics voiced their shock over the abrupt decision, days after senior American officials said Assad’s removal was no longer a priority, and pointed at Mr Trump’s previous assertions that a President would have to get Congressional approval to attack Syria.

Mr Tillerson said the strike was not “an emotional reaction” by the President, adding: “He came to the conclusion that we could not, yet again, turn away and turn a blind eye to what’s happened.”

The attack was a “one-off,” a US defence official told Reuters, meaning it was expected to be a single strike with no current plans for escalation.

Syrian state television hit out at “American aggression”, while Assad’s ally Iran also denounced the attack as a “destructive violation of international law”.

But the UK, Israel and other American allies joined the Syrian opposition welcomed the strikes.