Russia has warned that US cruise missile strikes on a Syrian air base could have "extremely serious" consequences, with the nation's prime minister saying the strikes were one step away from clashing with Russia's military.

Washington fired 59 cruise missiles at the Syrian Shayrat air base, almost completely destroying it, in response to a deadly chemical attack, described by Mr Trump as "barbaric".

The US, Britain and France have accused the Syrian leader of being behind the chemical attack that killed at least 70 people in rebel-held territory - though the regime has denied any involvement.

Syrian president Bashar al-Assad's office has denounced the US missile strike as "foolish" and "irresponsible".

"What America did is nothing but foolish and irresponsible behaviour, which only reveals its short-sightedness and political and military blindness to reality," the office said.

The strike killed nine civilians, including four children, state news agency SANA reported.

The al-Sharyat airfield. (Supplied)

The body of a plane burned as a result of the US strike. (AFP)

"Seven other civilians were wounded when a missile hit homes in Al-Manzul, four kilometres away from the Shayrat air base," SANA said.

The missiles: How much 59 Tomahawks cost the US

The strike was Mr Trump's biggest foreign policy decision since taking office in January and the kind of direct intervention in Syria's six-year-old civil war his predecessor Barack Obama avoided.

It has catapulted Washington into confrontation with Russia, which has advisers on the ground aiding its close ally Assad.

"We strongly condemn the illegitimate actions by the U.S. The consequences of this for regional and international stability could be extremely serious," Russia's deputy UN envoy, Vladimir Safronkov, told a meeting of the UN Security Council on Friday.

Russian Prime Minister Dmitry Medvedev charged that the US strikes were one step away from clashing with Russia's military.

US officials informed Russian forces ahead of the missile strikes and avoided hitting Russian personnel.

Satellite imagery suggests the base houses Russian special forces and helicopters, part of the Kremlin's effort to help Mr Assad fight Islamic State and other militant groups.

Mr Trump has frequently urged improved relations with Russia, strained under Mr Obama over Syria, Ukraine and other issues, but in launching the attacks on Thursday night he said action had to be taken against Mr Assad.

"Years of previous attempts at changing Assad's behaviour have all failed and failed very dramatically," Mr Trump said as he announced the attack on Thursday night from his Florida resort, Mar-a-Lago, where he was meeting Chinese President Xi Jinping.

US ambassador to the United Nations Nikki Haley said on Friday the Trump administration was ready to take further steps if needed.

Iran, which supports Assad and has been criticized by Mr Trump, condemned the strike, with President Hassan Rouhani saying it would bring "only destruction and danger to the region and the globe."

US officials called the intervention a "one-off" intended to deter future chemical weapons attacks and not an expansion of the US role in the Syrian war. Treasury Secretary Steve Mnuchin said the United States would announce additional sanctions on Syria in the near future but offered no specifics.

US allies from Asia, Europe and the Middle East expressed support for the attack, if sometimes cautiously.

Prime Minister Malcolm Turnbull backed the strike as a "swift and just response" to a war crime.

"This was a calibrated, proportionate and targeted response," he said.

Mr Turnbull said defence officials were briefed on the strike ahead of time, but no Australian personnel played a direct role in the attack.

Opposition Leader Bill Shorten also backed the US operation.

"We support the US sending a strong signal that these gas attacks should have never occurred - and they should never occur again," Mr Shorten said in a statement.

"The illegal and abhorrent use of chemical weapons by the Assad regime against its own people demanded a strong response."

US officials say the Tomahawk missiles were fired from two warships in the Mediterranean Sea. (AAP)

The action is likely to be interpreted as a signal to Russia, and countries such as North Korea, China and Iran where Mr Trump has faced foreign policy tests early in his presidency, of his willingness to use force.

The United States is now likely to be more aggressive in pursuing intelligence about Syria's suspected chemical weapons program. The Pentagon has also signalled interest in determining any Russian complicity.