President Assad met Vladimir Putin in Moscow in 2015. The Russian president is under pressure to cut ties with the Syrian regime

Britain is pushing western nations to impose new sanctions on Russia if it fails to cut ties with President Assad as the conflict over Syria escalates.

Russia and Iran warned jointly yesterday that they would “respond with force” if there were further US attacks in Syria. The Russian embassy in London suggested that if Moscow received an ultimatum from western powers this week it could lead to “real war”.

Boris Johnson, the foreign secretary, is demanding that western nations draw up “very punitive sanctions” in response to last week’s chemical attack on a rebel-held area, in which up to 90 people were killed. A paper on sanctions has been prepared for a meeting of G7 foreign ministers in Lucca, Italy, which starts today.

The foreign