Donald Trump has ordered the firing of 59 cruise missiles at a military target in Syria – a swift retaliatory blow for the President Bashar al-Assad’s alleged use of chemical weapons on civilians.

Reports said the barrage of Tomahawk missiles, fired from two US Navy vessels located in the Mediterranean Sea, targeted Al Shayrat air base in the central city of Homs, from which Syrian aircraft staged Tuesday’s chemical weapons attack, which killed up to 100 people. The missiles were launched between 8-9pm Eastern Standard Time, and there were no immediate details of how many people may have been killed.

Mr Trump had been weighing up the option of a military response to Mr Assad’s alleged use of chemical weapons on civilians in Idlib province earlier this week.

Footage shows tomahawk missiles being fired at Syria from the U.S Navy

“Tonight, I ordered a targeted military strike on the air base in Syria from where the chemical attack was launched,” Mr Trump said in remarks at his Mar-a-Lago estate in Florida. “It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.”

US President Donald Trump delivers a statement on Syria from the Mar-a-Lago estate in West Palm Beach, Florida (AFP/Getty)

Mr Trump, who had just minutes earlier been meeting with the Chinese premier, added: “Previous attempts at changing Assad’s behaviour have failed and failed very badly. Tonight, I’m calling all civilised nations to help us end the slaughter and bloodshed.”

The president had spoken earlier this week in the Rose Garden of the White House about how the images he had seen of dead children had changed his opinion on Mr Assad.

While on route to Florida to meet with the Chinese president, he was asked his thoughts on Mr Assad and he said “something should happen”.

Boris Johnson: Syrian chemical attack evidence points to Assad

CNN said that Mr Trump had been considering various options that were presented to him by his military chiefs. On Thursday evening, he met with defence secretary James Mattis and the Chairman of the Joint Chiefs of Staff, to discuss the different choices he had available.

The president was reportedly given a modest option, and a more extensive operation that would have destroyed much more infrastructure and probably resulted in much more loss of life.

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

As it was, it appears Mr Trump opted for the more moderate response, targeting the air-base believed to have been used for attack on Syrian civlians on Tuesday, but nothing that would appear sufficient, at least at this stage, to dislodge the Assad regime.

Initial reports suggested that the US struck one airstrip, fuel supplies and various Syrian aircraft. Nothing believed to contain chemicals weapons was targeted, and there were reportedly no Russian planes at the base.

Meanwhile, Syrian State televiosn lead with a story that was headlined: “American agression targets Syrian military targets with a number of missiles.”

Mr Trump’s options were complicated by a number of factors. Russian troops are on the ground in Syria and Washington would go to extremes to avoid casualties among its forces.

Indeed, reports from the Pentagon suggested that Russia had been warned in advance of the imminent operation.

There are also US special forces operating in Syria, along with anti-Assad rebels, a number of which have received arms, training and funding from the US and members of its coalition.

Alongside all of this, the US, along with Iraqi and Kurdish forces, are seeking to push back Isis, which makes its headquarters in the Syrian city of Raqqa.

It was reported late on Thursday, that US Secretary of State Rex Tillerson had told reporter that Russia had not been informed about the strike. However, steps were taken to ensure Russian planes or personnel were not hit.