The construction of a major port in eastern India threatens one of the world's largest mass nesting sites for endangered sea turtles, environmental activists said Saturday as they called for authorities to halt the project immediately.

The massive port, which will be the deepest in India, is less than 15 kilometers (9 miles) from Gahirmatha, a beach in the state of Orissa where hundreds of thousands of Olive ridley turtles come to make their nests in the sand.

Wildlife experts say there are only a few mass nesting beaches anywhere in the world, and losing the Gahirmatha beach would be catastrophic to the already fragile turtle population.

"This is India's most critical sea turtle habitat," said Belinda Wright, director of the Wildlife Protection Society of India. The mass nesting "is an extraordinary phenomenon that cannot be reproduced. If it's lost, it's gone forever."

The Dhamra port site does not overlap directly with the nesting beach, but its construction, along with the dredging and increased traffic in the area, would dramatically affect the offshore waters where the turtles breed, activists said.

The Dhamra port is a joint venture being built by Tata Steel, a subsidiary of Tata Group, the country's largest conglomerate, and the construction firm Larsen & Toubro Ltd. The port will handle shipments of coal, iron ore and limestone. The first phase of construction began last year and is expected to be finished in 2010.

Tata and the directors of the port say the project will not affect the turtle population, pointing to environmental impact studies conducted before construction began that support their claim.

Ashish Fernandes of Greenpeace dismissed those environmental reports as "shoddy" and said a 2007 survey commissioned by his group found rare amphibians and turtles at the port site.

"Tata needs to show that its commitment to the environment goes beyond mere lip service by halting the work immediately," said Fernandes.

The turtles did not nest in Orissa this year — likely because of the port construction, said Biswajit Mohanty of the Wildlife Society of Orissa.

Tata officials and Dhamra Port Company Ltd. did not respond immediately to a request for comment.