US Secretary of State Rex Tillerson has urged Russia to abandon its support for Syrian President Bashar al-Assad, amid growing tensions over a chemical weapons attack.

Key points: Tillerson says last week's actions stripped Mr Assad of all legitimacy

Tillerson says last week's actions stripped Mr Assad of all legitimacy Statement came shortly before leaving for Moscow, following urgent G7 meeting in Italy

Statement came shortly before leaving for Moscow, following urgent G7 meeting in Italy G7 nations ignore calls by US, Britain for new sanctions on Moscow

G7 nations ignore calls by US, Britain for new sanctions on Moscow Italian Foreign Minister says Russia must not be "pushed into a corner"

The statement came after a meeting of Group of Seven (G7) foreign ministers in Italy, following last week's apparent chemical weapons attack by Syrian government forces.

It also came just hours before Mr Tillerson arrived in Moscow on the first official trip to Russia by a member of US President Donald Trump's Cabinet.

Mr Tillerson said Russia must side with the United States and like-minded countries or embrace Syrian President Bashar al-Assad, Iran and the militant group Hezbollah.

He added that it was unclear whether Russia had failed to take seriously its obligation to rid Syria of chemical weapons, or had merely been incompetent.

But he said the distinction did not "much matter to the dead".

"We cannot let this happen again ... we want to relieve the suffering of the Syrian people," he told reporters shortly before leaving for Moscow.

"Russia can be a part of that future and play an important role, or Russia can maintain its alliance with this group, which we believe is not going to serve Russia's interests longer term."

Foreign ministers from G7 nations, including Mr Tillerson (L) and Boris Johnson (C), met in Italy. ( Twitter: @G7intheUS )

In a response, Russian President Vladimir Putin said Moscow would tolerate Western criticism of its role in Syria, but he hoped that attitudes would eventually soften.

When asked by a reporter if he expected more US missile strikes on Syria, Mr Putin said Mr Assad's enemies were planning to stage chemical weapons attacks to discredit the Syrian Government.

He did not offer any evidence for the assertion.

The US said its strike on the Syrian airbase near Homs on Friday was a one-off and not a strategic shift.

But the White House has also said Mr Trump could authorise more strikes if Syria uses chemical weapons again.

A satellite image shows the damage following a US airstrike on the Shayrat military airbase in Syria. ( AP: DigitalGlobe )

White House spokesman Sean Spicer suggested on Monday (local time) a lower bar for further US action, saying Washington could also retaliate if Syria used "barrel bombs" — oil drums packed with explosives dropped from aircraft.

"When you watch babies and children being gassed, and suffer under barrel bombs, you are instantaneously moved to action," he said.

"I think this president's made it very clear that if those actions were to continue, further action will definitely be considered by the United States."

Retaliating for barrel bombs would require a major shift in US policy since rebels say the weapons are used almost daily.

The Syrian Observatory for Human Rights, a British-based monitoring group, said Syrian warplanes dropped barrel bombs on rebel-held areas of Hama province on Tuesday (local time).

Syria has always denied using barrel bombs, though their use has been widely recorded by UN investigators. A source in the Syrian military denied it used them on Tuesday.

Tests confirm sarin use: Turkey

People are treated after the suspected gas attack in the town of Khan Sheikhoun. ( Reuters: Ammar Abdullah )

Meanwhile, Turkey's Health Ministry, which last week conducted autopsies on three of the victims of the chemical attack, told state news agency Anadolu their tests confirmed the use of sarin gas.

Health Minister Recep Akdag was quoted as saying that isopropyl methylphosphonic acid "has been identified in the blood and urine tests conducted on samples taken from the victims exposed to chemical warfare in Idlib".

The acid is formed from the degraded by-product of sarin reacting with other compounds.

Officials from the World Health Organisation and the Organisation for the Prohibition of Chemical Weapons participated in the autopsies.

Sorry, this video has expired Syrian allies say if the strikes allegedly shown in this footage are repeated, they will retaliate.

Russia must not be 'pushed into a corner'

The G7, consisting of Canada, France, Germany, Italy, Japan, the United Kingdom, and the United States, blames the Syrian military for the deadly chemical attack.

The ministers who met in Lucca, in central Italy, strongly supported the US missile strikes that targeted the Syrian air base believed to have been used to launch the attack.

But Italian Foreign Minister Angelino Alfano said Russia must not be "pushed into a corner" over Syria, as Britain and the US called for new sanctions on Moscow over its support of Mr Assad.

Mr Alfano said the US intervention had offered "a window of opportunity to construct a new positive condition for the political process in Syria".

But he said a political rather than military process was "the only solution", according to Italian news agency ANSA.

'Assad's reign is coming to an end': Tillerson

Mr Tillerson said the chemical attack had stripped Mr Assad of all legitimacy and it was clear the US saw no role for Mr Assad in Syria's future.

"It is clear to all of us that the reign of the Assad family is coming to an end," he said.

"We hope that the Russian Government concludes that they have aligned themselves with an unreliable partner in Bashar al-Assad."

Mr Tillerson said ceasefire talks that Russia and Iran had helped broker in the Kazakh capital, Astana, could generate momentum toward broader talks about a political transition if they succeeded.

The G7 countries were joined by diplomats from Muslim-majority nations including Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates.

On his visit to Moscow, Mr Tillerson plans to meet with Russian Foreign Minister Sergey Lavrov.

Mr Putin's spokesman said there were no scheduled plans for the President to meet Mr Tillerson, but Russian media have cited unnamed sources as saying such a meeting will happen.

ABC/AP/Reuters