LIMA (AFP) - Police detained four male tourists, two Australians and two Canadians, for several hours on Wednesday after filming and taking nude photos at Peru's famed Machu Picchu, local authorities said.

The culture ministry has put up signs at the world-famous Inca site warning that people taking their clothes off there was a "crime against culture" and would lead to immediate expulsion from the area.

The latest incidents came after Peru vowed to step up surveillance at its famous historical sites to counter a growing trend of tourists taking nude photos, including at Machu Picchu, and then posting them online.

The Australians, 22 and 24, used a camcorder and the Canadians, both 20, used an iPhone, police at a local police station told AFP.

It was not immediately clear if the four men acted together, separately or individually.

Machu Picchu, an Inca citadel from the 1400s, is a Unesco World Heritage Site.