Robbers staged an airborne raid on a Swedish cash depot late yesterday, landing a helicopter on the roof before ransacking the building.

Police were unable to chase them because they feared their own helicopter base in Stockholm had been booby-trapped with explosives by the gang.

The police said the robbers jumped onto the roof of the depot belonging to security firm G4S. Staff were inside, but no one was injured.

Bystander Bjorn Lockstrom said he saw a grey helicopter hovering above the building for about 15 minutes. "Two men hoisted themselves down," he said. "I also saw them when they hoisted up money, too." Police refused to say how much had been stolen.

They later found an abandoned helicopter near a lake north of Stockholm. It had been reported stolen and was believed to be the one used by the robbers.

Sweden has seen a series of spectacular robberies in recent years. Last year a group broke into a mail processing centre in Goteborg, paralysing large parts of Sweden's second-largest city after spreading spikes on the roads, burning out cars in several different areas and leaving suspected bombs in the centre.

In 2006, Goteborg's international airport was partially closed after a group of masked men crashed through a gate and held up luggage handlers as they were unloading crates of foreign currency worth 1.1 million dollars from a passenger aircraft.

Four years earlier, robbers pulled off a similar raid at Stockholm's Arlanda Airport, when staff were loading foreign currency onto an aircraft.