The United States and the United Kingdom will push for increased sanctions against Russia if it does not withdraw its backing for Syrian President Bashar al-Assad.

The two countries are to make their case for further punitive action against Russia at the G7 meeting in Italy on Tuesday (11 April) following a suspected chemical weapons attack in Syria's Khan Sheikhoun last week.

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Both nations said Assad was responsible for the attack, but countries such as Italy, France and Germany have indicated they would not back the measure.

British Foreign Secretary Boris Johnson finalised the intended approach with US Secretary of State Rex Tillerson on Monday (10 April), on the eve of Tillerson's trip to Moscow.

"We will be discussing the possibility of further sanctions, certainly on some of the Syrian military figures and indeed on some of the Russian military figures who have been involved in co-ordinating the Syrian military efforts and are thereby contaminated by the appalling behaviour of the Assad regime," Johnson said, according to the Times.

"They have a choice. That choice is to stick like glue to the Assad regime ... or to work with the rest of the world [for] a political solution."

Prime Minister Theresa May also discussed the prospect of increased Russian sanctions with US President Donald Trump via telephone on Monday.

"The prime minister and the president agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest," a Downing Street spokesperson said.

"They agreed that US Secretary of State Tillerson's visit to Moscow this week provides an opportunity to make progress towards a solution which will deliver a lasting political settlement."

Germany said sanctions would damage efforts to negotiate peace in Syria and Italy confirmed it was not considering further sanctions at this moment in time.

France is thought to be unlikely to push for penalties until its presidential elections are concluded.

The Syrian government and Russia have both denied playing any part in the chemical weapon attack and have called for an independent investigation, which is backed by Iran, China, Bolivia and Scotland.