During the early hours of Friday, Syrian time, the United States launched more than 50 Tomahawk missiles at the al-Shayrat military airfield near Homs, an airbase that is under the control of the Assad government.

Chemical weapons were launched from this same airbase on Tuesday, killing at least 80 people in the town of Khan Sheikhoun in Syria, including some 30 children

The Syrian government denied responsibility for the chemical attack. The Russian government said chemicals leaked when the Syrian government conducted an air strike on a rebel-held chemical weapons depot.

Trump said the strike was designed "to prevent and deter the spread of deadly chemical weapons".

He said there was no doubt that Syria was behind the chemical attack.

"Years of previous attempts at changing Assad's behaviour have all failed," he said. He called on other nations to end terrorism.

"We ask for God's wisdom as we face this challenge," he said.

The latest reports suggest at least nine civilians, including four children, were killed in the missile attack, which also caused extensive damage to the air base. Agence France-Presse had reported that at least four Syrian soldiers had been killed. Russian lawmaker Dmitry Sablin was quoted saying that no Russians were hurt.

Prime Minister Malcolm Turnbull said he "strongly supports" the strikes.

He said on Friday that Australian officials were briefed about the strike before it took place.

"The retribution has been proportionate and it has been swift, and we support the US in that swift action," he said.

He said that Australia was not at war with Assad and the strikes were not designed to overturn the Syrian president's regime.

"Civilians, women, children, babies – it was a crime that called out for a swift response," he said.

He has previously called for a strong response to the deadly chemical attack.

Opposition leader Bill Shorten also welcomed the action. "We support the US sending a strong signal that these gas attacks should have never occurred - and they should never occur again," he said.

Russian is one of Assad's main backers, the other is Iran. Russian President Vladimir Putin says the missile strikes broke international law and would "inflict major damage" on US-Russia relations.

"Putin views the US strikes on Syria as aggression against a sovereign state in violation of the norms of international law and on a made-up up pretext," Mr Putin's spokesman was reported saying.

"Washington's step will inflict major damage on US-Russia ties." It has been reported that Russia is calling call for an urgent meeting of the U.N. Security Council to discuss the situation.

Russia's coalition with the United States on the Syria crisis looks shaky, according to a senior Russian lawmaker. Konstantin Kosachev, who is chairman of the Russian parliament's international affairs committee, said the strikes were intended to "stamp an earlier verdict about al-Assad's responsibility for a chemical attack in Idlib with gunpowder", according to a report by the Interfax news agency.

Russia has since said it will suspend a deal with the US to prevent mid-air collisions over Syria, according to the Associated Press.

Under the memorandum, signed after Russia launched an air campaign in Syria in September 2015, Russia and the US had exchanged information about their flights to avoid incidents in the crowded skies over Syria.

Iran reportedly denounced the attack as "destructive and dangerous".

The attack was described as "American aggression" by Syrian state TV.

Israeli Prime Minister Benjamin Netanyahu said Trump had sent a strong message "in both word and action" that the use of chemical weapons would not be tolerated.

The Syrian National Coalition opposition group welcomed the strikes, saying it hoped they would continue. "We hope for the continuation of the strikes in order to prevent the regime from using its planes to launch any new air raids or going back to using internationally banned weapons," the Coalition's media official Ahmad Ramadan said.

A Downing Street spokesman said the British government "fully supports US military action in Syria." He said that the attack was "was an appropriate response to the barbaric chemical weapons attack launched by the Syrian regime, and is intended to deter further attacks." The Polish government said they supported all actions to end the humanitarian crisis in Syria. But Iran slammed the attack.

"Iran ... condemns use of chemical weapons ... but at the same time believes it is dangerous, destructive and violation of international laws to use it as an excuse to take unilateral actions," the Students News Agency ISNA quoted Foreign Ministry spokesman Bahram Qasemi as saying.

"Iran strongly condemns any such unilateral strikes ... Such measures will strengthen terrorists in Syria ... and will complicate the situation in Syria and the region."

A Pentagon statement said "he strike was a proportional response to Assad's heinous act" "Shayrat Airfield was used to store chemical weapons," it said.

US Republican Senator John McCain said the strikes sent an important message that the US would not "stand by" as Assad, aided by Russia, "slaughters innocent Syrians."

It could be a one off attack. Reuters is quoting a US defense official saying the Tomahawk strike was a "one off". Turnbull made a similar statement at his Sydney press conference earlier today.

The UN Security Council may convene another emergency meeting. The world is expecting Russia and Iran to make a statement.

On Thursday, the United States signalled it could take a more aggressive response to the chemical attack.

When asked if the attack on a Syrian hospital crossed a "red line", Trump said "it crossed a lot of lines". "That crosses many, many lines, beyond red lines," he said.

At a sitting of the United Nations Security Council an hour earlier, US Ambassador Nikki Haley stood up at her desk to show diplomats photos of dying children and accused Russia, which backs Syria, of pushing a false narrative blaming rebel forces for the attack.