A decision late on Wednesday by US District Court Judge Paul Rosenblatt would allow a $25 million upgrade to begin at the Snowbowl in the San Francisco Peaks. The 777-acre (314.5-hectare) facility is on federal forest land.

In a statement, the Navajo Nation pledged to "do whatever it takes" to overturn the ruling.

"This decision further eviscerates the rights of Native Americans to protect sacred lands that are essential to their belief systems," said Howard Shanker, an attorney representing three tribes and three activist groups in the case.

"This is a direct affront to traditional Native beliefs and one more nail in the coffin of all the tribes throughout the country (and their) ability to survive intact."

Rosenblatt upheld a US Forest Service decision, ruling that opponents failed to show that the upgrade plans would interfere with the free exercise of religion.

Snowbowl General Manager J.R. Murray said he was pleased with the ruling and hoped to start the work as soon as possible.

Shanker told Reuters on Thursday he plans by the end of next week to appeal the ruling and seek an injunction to try to stop work from beginning at the popular ski resort, a fixture in the northern Arizona area since 1938.

The Navajo Nation, which has an estimated 300,000 tribal members in Arizona, New Mexico and Utah, had joined with several other tribes and environmental groups to go to court following approval of the resort's plan in March by US Forest Service officials.

At the heart of the appeals was the claim that such plans would desecrate sacred land that is important to several tribes' religious practices and beliefs.

In the 62-page ruling, Rosenblatt wrote that the Forest Service decision did not bar "access, use or ritual practice on any part of the Peaks. The decision does not coerce individuals into acting contrary to their religious beliefs, nor does it penalize anyone for practicing his or her religion."