Summer climbers are ignoring the seasonal closure of a sacred Native American site.

During June, visitors are asked not to climb Devils Tower - a Wyoming National Monument - in order to respect American Indian ceremonies.

But the closure is voluntary and the number of those scaling the site has been increasing in recent years.

The rock formation is part of the Black Hills mountain chain, near Belle Fourche River. It rises 386m above the surrounding area and was declared the first National Monument of the United States by President Theodore Roosevelt.

In the early 1990s, a park management plan was jointly developed by a group of American Indians and climbing representatives. The climbers wanted to show that their community respected the site and the tribal representatives did not think a full closure reflected the ethos of the sacred space.

The voluntary closure was agreed upon in the spirit of compromise, explains Tim Reid, Superintendent of Devils Tower National Monument.

Speaking to Wyoming Public Media, Reid said: “June was the selected month. And they wanted people to abstain from climbing out of respect for the sacred site status, and the cultural significance of the tower to 25-plus tribes in the intermountain area.”

When it was first implemented in June 1995, the scheme worked very successfully, reducing the number of climbers from 1,200 to 167, according to data from the National Parks Service.

But climbers have begun to flout the voluntary code in recent years. Last year, 373 people climbed the tower in June.

Reid said: “In the last five years, there’s been a steady incremental increase in the number of climbers in June that is not connected to just the steady overall increase of visitation at the monument.

“Largely, the bulk of June climbing is done by relatively local or regional climbers, who for whatever reason find it personally acceptable to climb in June.”

Waylon Black Crow Senior, one of many Native Americans on an annual visit to the sacred site, said the practice upset him. He said: “We see them climbing up there. And all we can do is watch.”