British Foreign Secretary Boris Johnson has cancelled a scheduled visit to Moscow due developments in Syria where a suspected poison gas attack in a rebel-held area prompted the United States to launch missile strikes on a Syrian airbase.

"Developments in Syria have changed the situation fundamentally," Johnson said in a statement on Saturday.

The foreign secretary, who was due to travel to Moscow on Monday, called Russia's defence of the Syrian government in the wake of the suspected chemical attack 'deplorable'.

He said his priority was "to continue contact with the US and others" to build international support for a ceasefire.

Syria's army said six people were killed in the early hours of Friday morning after the US fired nearly 60 Tomahawk missiles at Shayrat Air Base, in retaliation for a suspected chemical weapons attack on a rebel-held town of Khan Sheikhoun that killed scores of civilians earlier this week.

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Johnson said he had discussed his plans "in detail" with US Secretary of State Rex Tillerson, adding that Tillerson would still visit Moscow as planned following the G7 summit on April 10-11 to "deliver that clear and coordinated message to the Russians".

Johnson also called on Russia, a key ally of Syrian President Bashar al-Assad, to do "everything possible to bring a political settlement in Syria ... to ensure that the shocking events of the last week are never repeated".

"We deplore Russia's continued defence of the Assad regime even after the chemical weapons attack on innocent civilians," he said.

Johnson expressed his support to the US for Friday's missile barrage.

The US strike was in response to a suspected chemical weapons attack on the rebel-held town of Khan Sheikhoun that killed at least 88 people, including 29 children, according to the UK-based Syrian Observatory for Human Rights.

The Syrian government and Moscow have denied that Syrian forces were behind the gas attack, but Western countries have dismissed their explanation that chemicals leaked from a rebel weapons depot after an air strike as not credible.

Russia, alongside Iran, condemned Friday's US strike, denouncing it as a "flagrant violation of international law and an act of aggression".

The US said on Saturday that the strike was "fully justified", adding it was "prepared to do more" if necessary.