Peruvian firefighters were battling to contain a forest fire near the Incan ruins of Machu Picchu as the blaze threatened to close in on the ancient city high in the Andean mountains on Thursday.

The fire, which had engulfed an area about half the size of Vatican City, was started on Tuesday by farmers burning grass and debris to prepare to sow new crops.

As of Wednesday, about 20 hectares (49 acres) had been affected by the fire, the mayor of the nearby city of Cusco said.

Machu Picchu, a complex of stone structures sitting atop a mountain, was built more than 500 years ago by the Incas, whose empire controlled large swathes of South America from what is today southern Ecuador to central Chile.

The fire's remoteness has hindered firefighters' efforts.

"We have already been fighting the forest fire for two days and it has not been possible to get it under control, given the area is quite inaccessible," said Roberto Abarca, director of the Cusco risk management and security office.

The breathtaking ruins, which have made the surrounding Cusco region Peru's top tourist destination, are considered one of the new seven wonders of the world.