Vladimir Putin

World leaders will gather in St Petersburg on Thursday for what has transformed into an international showdown with Vladimir Putin threatening to send a missile shield to Syria if the US launches an attack without the authority of the United Nations.

The G20 summit, hosted by Putin, had been expected to focus on the world economy and growth, but will now be dominated by the Middle East crisis, even if the formal agenda remains fixed on the slowdown of growth in emerging markets.

Barack Obama, speaking during a stopover in Sweden before the summit, denied his political credibility was at stake but admitted relations with Russia had hit a wall. He insisted he had not set the red lines requiring a military response if the Syrian government deployed chemical weapons.

“The world set a red line when governments representing 98% of the world’s population said the use of chemical weapons was abhorrent and passed a treaty forbidding their use even when countries are engaged in war,” he said. “That was not something I just kind of made up, I did not pluck it out of thin air.”