The United States has fired a barrage of cruise missiles into Syria in retaliation for this week's chemical weapons attack against civilians.

Key points: About 60 US Tomahawk missiles targeted Syrian airbase near Homs

About 60 US Tomahawk missiles targeted Syrian airbase near Homs Donald Trump said missile attack in the nation's "vital national security interest"

Donald Trump said missile attack in the nation's "vital national security interest" Syria's ally Russia was given advance warning

It was the first direct American assault on the Syrian Government, and Donald Trump's most dramatic military order since becoming president.

About 60 US Tomahawk missiles, fired from warships in the Mediterranean Sea, targeted an air base in retaliation for a chemical weapons attack that American officials believe Syrian Government aircraft launched with a nerve agent, possibly sarin.

The targets at the Government-controlled Shayrat base, in central Syria, were an airstrip, aircraft and fuel stations, a US official said.

The base was where US officials say the Syrian military planes that dropped the chemicals had taken off.

The missiles hit early Friday morning, local time, with the US military claiming it appeared the strikes severely damaged or destroyed Syrian aircraft as well as support infrastructure and equipment.

Sorry, this video has expired Syrian Foreign Minister denies Syrian use of chemical weapons

The Russian Defence Ministry said the attack destroyed six planes in repair hangers, but said only 23 of the rockets hit the target and it was not clear where the other 36 landed, according to Russian news agencies.

In a statement, the office of Syrian President Bashar al-Assad called the attack "reckless" and "irresponsible", saying it showed Washington was "naively pulled behind a false propaganda campaign".

The statement said the strike was "short-sighted" and reflected a continuation of policy regardless of administration that was based on targeting and "subjugating people".

The local governor of Homs province said seven people were killed in the strike. Russian news agencies quoted the country's Defence Ministry as saying four people died.

The British-based Syrian Observatory for Human Rights said the attack killed at least four Syrian soldiers, including a general, in its report which it said was based on its own sources.

"The use of chemical weapons against innocent people will not be tolerated," the Pentagon said in a statement, adding the strike was intended to deter the regime from using chemical weapons again.

The President's national security adviser, General HR McMaster, said Mr Trump was given three options for possible responses to the chemical attack and told advisers to focus on two. He made a decision the day before the strikes.

Sorry, this video has expired Footage allegedly shows US missile strikes on Syrian air base.

State-controlled Syrian television station al-Ikhbariyah aired what it described as exclusive footage showing the aftermath of the attack.

The video shows a fast sequence of orange flashes that light the dark sky in the distance before the crack of dawn.

Trump says attack was in 'vital security interest' of the US

Sorry, this video has expired Donald Trump speaks after the missile attack

Speaking after news of the launch emerged, Mr Trump said he called on "all civilised nations" to seek to end the bloodshed and slaughter in Syria.

He said the US missile attack was in the nation's "vital national security interest", arguing that the United States must "prevent and deter the spread and use of deadly chemical weapons".

He said there was "no dispute that Syria used banned chemical weapons" in Tuesday's attack on the town of Khan Sheikhoun.

"Assad choked out 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 barbaric attack," Mr Trump said in Florida, where he was holding talks with China's leader Xi Jinping.

He said previous attempts at getting Mr Assad to change his behaviour had failed.

The surprise strike marked a striking reversal for Mr Trump, who warned as a candidate against the US getting pulled into the Syrian civil war, now in its seventh year.

But the President said he was moved by the photos of children killed in the chemical attack, calling it a "disgrace to humanity" that crossed "a lot of lines."

US Secretary of State Rex Tillerson said the attack showed the President "is willing to take decisive action when called for".

"I think it does demonstrate that President Trump is willing to act when governments and actors cross the line and cross the line on violating commitments they've made and cross the line in the most heinous of ways," he said.

The President did not announce the attacks in advance, though he and other national security officials ratcheted up their warnings to the Syrian Government throughout the day Thursday.

Sorry, this video has expired GRAPHIC CONTENT WARNING: Suspected Syrian Government chemical attack kills more than 50.

Strike 'aggression against a sovereign nation', Putin says

US officials have placed some of the blame on Russia, one of Syria's most important benefactors, for the chemical attack. Mr Tillerson, in Florida with Mr Trump, said Moscow had failed to live up to a 2013 agreement that was intended to strip Syria of its chemical weapons stockpiles.

"Either Russia has been complicit or Russia has been simply incompetent in its ability to deliver on its end of the agreement," Mr Tillerson said.

But Russian President Vladimir Putin has reportedly hit back, saying the US strike broke international law and seriously hurt US-Russia relations.

News agencies citing Mr Putin's spokesman Dmitry Peskov said he regarded the US action as "aggression against a sovereign nation" on a "made-up pretext" and as a cynical attempt to distract the world from civilian deaths in Iraq.

Mr Peskov was quoted as saying Russia did not believe that Syria possessed chemical weapons and the US move would inevitably create a serious obstacle to creating an international coalition to fight terrorism, an idea Mr Putin has repeatedly pushed.

Fact file: Tomahawk missile The Tomahawk is an all-weather, long-range cruise missile used for land attack warfare

The Tomahawk is an all-weather, long-range cruise missile used for land attack warfare They fly low, at less than the speed of sound, and have several guidance systems

They fly low, at less than the speed of sound, and have several guidance systems First used against Iraqi targets during Operation Desert Storm in 1991

First used against Iraqi targets during Operation Desert Storm in 1991 Launched from warships and powered by a jet engine

Launched from warships and powered by a jet engine The missiles were also used during the fighting in Libya Source: United States Navy

Russia said it would now suspend a deal with the US to help prevent mid-air collisions over Syria.

The deal meant Russia and the US would exchange information about their flights to avoid incidents in the crowded skies over Syria.

Russian Defense Ministry spokesman Major General Igor Konashenkov said a "complex of measures" to strengthen Syrian air defences would be done shortly to help "protect the most sensitive Syrian infrastructure facilities".

Viktor Ozerov, the head of the defence and security committee at the Russian Upper House of Parliament, said Russia would also call for an urgent meeting of the UN Security Council.

"This could be viewed as an act of aggression of the US against a UN nation," he was quoted as saying.

The Pentagon said Russia was given advance notice of the strikes, via an "established deconfliction line" the two countries share, adding the missiles did not target areas Russians were occupying.

The United States also told Australia of the strikes before they took place. Defence sources told the ABC Australia was advised of the strikes "a couple of hours" before they happened.

A Syrian opposition group, the Syrian Coalition, welcomed the US attack, saying it puts an end to an age of "impunity" and should be just the beginning.

Strike sends vitally important message, Turnbull says

Sorry, this video has expired Turnbull reiterates support for US strike on Syria

Prime Minister Malcolm Turnbull this afternoon told reporters that while Australia was not involved in the strike, it supported the action.

"This is a vitally important signal, a vitally important message, that we will not tolerate — the world will not tolerate — the use of these chemical weapons," he said.

"The retribution has been proportionate and it has been swift. We support the United States in that swift action."

Defence Minister Marise Payne said US Secretary of Defence James Mattis advised her before the strike.

She said in light of the attack, Australia was reviewing its protection of military assets in the region.

"As you know, Australia's air task group is confined to operations in eastern Syria, including in the vicinity of Raqqa, where we continue to target Daesh," she said.

"Australia has also taken appropriate measures in light of this operation to review our force protection arrangements in the Middle East."

The government-controlled Shayrat airbase seen via satellite. ( Reuters: US Department of Defence )

Mr Turnbull refused to say if the US strike would be a one-off attack on Syria, but stressed it was not indicative of a war against the Assad regime.

"It is a proportionate and calibrated response designed to prevent that airfield being used to deliver chemical weapons again," he said.

"You can imagine, 59 cruise missiles is a substantial attack on that airfield. But we are not at war with the Assad regime and United States have made it clear that they are not seeking to overthrow the Assad regime."

A military official quoted on Syrian TV said an air base in central Syria was hit early on Friday, causing material damage. Another statement, also attributed to an unnamed official, referred to "losses." The officials did not elaborate.

ABC/wires