Rex Tillerson, the U.S. Secretary of State, will fly to Moscow later this week with an ultimatum from the G7 demanding that Putin withdraws his armed forces and ends his support for President Bashar al-Assad. Threats of new sanctions against Russia could be made if he does not comply, according to The Daily Telegraph.

But alongside the hardline approach being taken by America and its Western allies, diplomats will be working behind the scenes to offer Mr. Putin a way to regain his place at the G7, something he is said to covet.

Read alsoRussia, Iran say U.S. crossed "red lines" in SyriaRussia was expelled from what was the G8 in 2014 as a direct response to its annexation of Crimea. Sergey Lavrov, Russia's foreign minister, tried to shrug off the punishment at the time, saying the G20, of which Russia remains a member, was far more important.

British and American diplomats, however, believe that Mr. Putin is desperate to rejoin the G7, which consists of the U.S., the UK, France, Germany, Canada, Italy and Japan. They also believe that his enthusiasm for the Assad regime is waning, and that he is looking for a dignified way out.