Britain has backed the US missile strike on a Syrian air base as an "appropriate response" to Bashar Assad regime's "barbaric" chemical attack.
The Government has offered its full support to US president Donald Trump's targeted assault on the base from where he said a devastating nerve agent strike on civilians was launched.
The surprise barrage of 59 cruise missiles in the early hours of Friday, UK time, was the first direct US attack on the Syrian government.
Mr Trump was reacting to the attack on Tuesday that killed at least 72 people, including 20 children, which he said was launched by Syrian president Assad.
On Friday morning, a No 10 spokeswoman said: "Overnight, the US has taken military action against the Syrian regime, targeting the airfield in Shayrat which was used to launch the chemical weapons attack earlier this week.
"The UK Government fully supports the US action, which we believe was an appropriate response to the barbaric chemical weapons attack launched by the Syrian regime, and is intended to deter further attacks."
Speaking from Florida, Mr Trump announced his strike in an emotional message to the public in which he evoked images of children dying.
"Using a deadly nerve agent, Assad choked out the lives of helpless men, women and children. It was a slow and brutal death for so many," he said.
"Even beautiful babies were cruelly murdered in this very barbaric attack.
"Tonight, I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched.
"It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.
"There can be no dispute that Syria used banned chemical weapons."
US Tomahawk missiles, launched from two warships in the Mediterranean Sea, targeted airstrips, hangars, control towers and ammunition areas in Sharyat, central Syria, according to officials.
They suspect a mixture of chlorine and a nerve agent were used in Syria's attack on the largely opposition-held Idlib province.
Mr Trump said the latest action was in the "vital national security interest", adding that the US must "prevent and deter the spread and use of deadly chemical weapons".
He also called for other "civilised nations" to join efforts "seeking to end the slaughter and bloodshed in Syria".
Britain had been leading renewed calls for diplomatic action in response to the earlier chemical attack.
The US, UK and France had brought a resolution before the United Nations Security Council, demanding an investigation.
Earlier on Wednesday, Downing Street had played down the prospect of military action, insisting "nobody is talking" about an armed response to the atrocity.
Syrian state TV went on to report missile attacks on a number of military targets, calling them an act of "aggression" which had led to "losses".
Pentagon spokesman Captain Jeff Davis said the strike was a "proportional response to Assad's heinous act".
It succeeded in "reducing the Syrian government's ability to deliver chemical weapons" by severely damaging or destroying aircraft, according to initial indications, he added.
He also said Russian forces were alerted ahead of the strike to minimise casualties at the airfield.
US secretary of state Rex Tillerson said Russia had "failed" to deliver its commitment to secure Syria's chemical weapons, saying it had been either complicit or "simply incompetent".
Major Jamil al-Saleh, a Syrian opposition commander whose district has been hit by chemical weapons, welcomed the US attack and hopes it will be a "turning point" in the six-year civil war.
The Syrian Coalition opposition group also backed the move, with senior official Ahmad Ramadan urging Mr Trump to "hit the snake's head".