Russia and the United States conducted the biggest spy swap since the Cold War, trading agents in Vienna airport in the climax to an espionage drama that had threatened improving ties.

Two jets - one Russian, one American - parked side by side for around 90 minutes as vehicles shuttled between them. The agents changed places under the cover of aeroplane gangways as waves of heat rose from the blistering blacktop.

The Russian plane then took off, followed by the US jet in an echo of Soviet-era spy trades across the Iron Curtain in central Europe. Officials in Vienna maintained a strict news blackout.

But the US Justice Department announced shortly after the take-off that the exchange of 10 agents released by Washington and four freed by Moscow had been successfully completed.

The jet landed at Domodedovo airport outside Moscow a few hours later, and a car pulled up at the foot of the gangway.

But the exchange may add fuel to Republican accusations that Obama is being too soft on Moscow. A key 11th suspect named by US authorities disappeared after being granted bail following his arrest in Cyprus.