The threat of direct Russian-American confrontation in Syria escalated on Monday after Moscow said it would treat any plane from the US-led coalition flying west of the Euphrates river as a potential target.

Russia said it was responding to US planes shooting down a Syrian air force jet on Sunday. The US said its planes had acted to defend US-backed forces seeking to capture the Islamic State capital of Raqqa in north-east Syria.

It was the first such US attack on a Syrian warplane since the start of the country’s civil war six years ago.

Russia’s deputy foreign minister, Sergei Ryabkov, said the US strike “has to be seen as a continuation of America’s line to disregard the norms of international law.

“What is this if not an act of aggression? Itwith Syria/Iran/Russia in 45 days. Trump is quietly starting a new war that Congress has not declared. Red alert.”

In a sign of how complex the Syrian peace process has become, Russian-sponsored peace talks in Astana, Kazakhstan, are scheduled to resume on the same day – 10 July – as talks convened by the UN in Geneva.

The Russian foreign minister, Sergei Lavrov, announced the date on Monday in the knowledge that it would coincide with the UN schedule. He also said that the UN’s Syria envoy, Staffan de Mistura, would take part.

A spokesman for de Mistura said “the subject is currently being discussed”.