The United States today launched a massive military strike on a Syrian air base in retaliation for a "barbaric" chemical attack on civilians as President Donald Trump appealed to all "civilised nations" to join America to end the "bloodshed" in the war-torn country.

On President Trump's orders, US warships launched between 50-60 Tomahawk cruise missiles at a Syrian government airbase where the warplanes that carried out the chemical attacks were based, US officials said.

Speaking inside his Mar-a-Lago resort where he met Chinese President Xi Jinping, Trump said he ordered targeted missile strikes at a Syrian airfield from where the chemical attack was launched."

Describing the chemical attack on innocent civilians as "horrible", Trump said President Bashar al-Assad took the lives of helpless men, women and children. "It was a slow and brutal death for so many. Even beautiful babies were cruelly murdered in this very barbaric attack. No child of God should ever suffer such horror," he said.

"It is in vital national security interest of the United States to prevent and deter use of deadly chemical weapons," Trump told a pool of reporters. "There can be no dispute that Syria used the banned chemical weapon violated its obligations under the chemicals weapons convention and ignored the urging of the UN Security Council," he said.

"Years of previous attempts at changing Assad's behaviour have all failed and failed very dramatically. As a result the refugee crisis continue to deepen and the region continues to destabilise threatening the US and its allies. I call on all civilised nations to join us seeking to end this slaughter and bloodshed in Syria and also to end terrorism of all kinds and types," he said.

"We asked for God’s wisdom as we face the challenge of our very troubled world. We pray for the lives of the wounded and for the souls of those who passed. And we hope as long as America stands for justice and peace and harmony will in the end prevail. Good night and God Bless America and the entire world," Trump said in his brief remarks.

Pentagon Press Secretary Capt Jeff Davis said Russian forces were notified in advance of the strike using the established deconfliction line.  “US military planners took precautions to minimise risk to Russian or Syrian personnel located at the airfield," he.

The Pentagon said the missile strike that targeted Shayrat Airfield in Homs governorate was a proportional response to Assad's heinous act. "The strike was a proportional response to Assad's heinous act. Shayrat Airfield was used to store chemical weapons and Syrian air forces. The US intelligence community assesses that aircraft from Shayrat conducted the chemical weapons attack on April 4," Pentagon Press Secretary Capt Jeff Davis said.

The strike was intended to deter the regime from using chemical weapons again, he said. He said the US informed Russia ahead of the Syria strike. "Russian forces were notified in advance of the strike using the established deconfliction line. US military planners took precautions to minimise risk to Russian or Syrian personnel located at the airfield," Davis said.

The suspected chemical attack left at least 86 people dead in the rebel-held town of Khan Sheikhun in northwestern Syria. The deaths sparked international outrage with many pointing the finger at the government of President Assad. The Syrian army on Tuesday denied that it had used chemical weapons against Khan Sheikhun. The airstrike has been welcomed by top American lawmakers.

"Unlike the previous administration, President Trump confronted a pivotal moment in Syria and took action," said Senators John McCain and Lindsay Graham in a joint statement. "Acting on the orders of their commander-in-chief, they have sent an important message the US will no longer stand idle as Assad, aided and abetted by Putin’s Russia, slaughters innocent Syrians with chemical weapons and barrel bombs," they said.

Applauding Trump for taking such an action, Senator Bob Corker, Chairman of the Senate Foreign Relations Committee said it is critical that Assad knows he will no longer enjoy impunity for his horrific crimes against his own citizens, and this proportional step was appropriate.

"I support the President's decision to take military action in Syria. My hope is that this action will help deter future atrocities by the Assad regime," said Congressman Mac Thornberry, Chairman of the House Armed Services Committee.

"These military strikes against Assad’s arsenal send a clear signal that the United States will stand up for internationally accepted norms and rules against the use of chemical weapons," said Senator Ben Cardin, Ranking Member of the Senate Foreign Relations Committee.

"The Joint Investigative Mechanism of the United Nations-Organisation for the Prohibition of Chemical Weapons has confirmed multiple times that Assad regime forces have used chemical weapons against the Syrian people," he said. Senate Democratic Whip Dick Durbin said this was a measured response to the Syrian nerve gas atrocity.

"Any further action will require close scrutiny by Congress, and any escalation beyond airstrikes or missile strikes will require engaging the American people in that decision," he said.