At least 41 people, most of them police officers, have been killed after a bus fell off a bridge in north-west Argentina, officials say.

The bus was carrying at least 50 people, most of them members of Argentina's border police, when it came off the road and fell about 20m (65ft) into a riverbed.

Emergency workers are trying to free a number of trapped passengers.

They said that so far, six passengers had been rescued from the wreck.

The bus came off the road at about 02:00 local time (05:00 GMT) near the town of Rosario del Fuente in Salta province, police officials said.

It was one of three police buses travelling on national highway 34.

A local police statement said there were indications that one of the bus's tyres had burst and caused the driver to lose control of the vehicle.

Speaking at a public event, President Mauricio Macri, expressed his condolences.