PEREVOZNAYA, Russia — Stretching from Lake Baikal to the Sea of Japan, the first trans-Siberian oil pipeline is to run 4,150 kilometers - more than three times as long as the trans-Alaska pipeline. At a cost of $15.5 billion, it looms as modern Russia's biggest infrastructure investment, President Vladimir Putin's answer to the Trans-Siberian Railway of the czars.

Because China and Japan both rely on the Middle East for about 85 percent of their oil imports, both economic giants competed fiercely over what could be the world's longest and most expensive oil pipeline. Trumping China with a more generous financing offer, Japan, the world's second-largest oil importer, hopes that the pipe, 1.2 meters, or four feet, in diameter, will bind it to Russia, the world's second-largest oil exporter.

It may be a decade before the 2,580-mile pipeline is completed. But the line would represent an increase of about one-third in Russia's oil pipeline export capacity and would signify a major Russian shift toward the Pacific, where oil could be sold to any country, including the United States.

The project faces major hurdles. There are no guarantees that there will be enough oil to fill the pipeline, although Russia has as much as 67 billion barrels of untapped oil reserves along the pipeline route. When the oil reaches the Sea of Japan, there are no public commitments binding Russia to sell it to Japan, whose ports are only a day's sail away.

And Russia's last-minute switch of the Pacific terminal site from an existing oil port to this pristine bay is already putting Japanese banks in the cross hairs of a growing global environmental protest movement.

The Kremlin has vowed not to contribute state funds for the pipeline construction. Instead, Transneft, the state pipeline monopoly, has been told by Moscow to find its own financing, preferably on international markets.

Foreign banks, especially the Japan Bank for International Cooperation, will increasingly face questions over the pipeline's future profitability and over Moscow's little-publicized decision in December to switch the pipeline terminus to Perevoznaya, a tranquil bay known for its beaches and wild nature, from Vostochny, Russia's main industrial port in the Pacific.

In the summer, this bay's sandy beaches and its warm waters are frequented by ferries filled with beachgoers from Vladivostok, 16 kilometers to the east. In the winter, this frozen semi-wilderness is home to some of the last 30 to 40 Amur leopards in the world. This black-spotted cat is one of 50 endangered species found only in this corner of Pacific Russia.

To get to the terminal and planned oil refinery, dozens of tankers would steam daily past Russia's only maritime nature reserve, a collection of 11 islands prized for 3,000 species. Navigating an eight-kilometer-wide channel littered with more small islands, the tankers would then enter a maritime cul-de-sac with shallow waters and fragile ecology.

This winter, U.S. and European environmentalists are organizing an international campaign to persuade Moscow to go back to its original plan to build the oil pipeline to Vostochny, a modern industrial port and railhead built 30 years ago next to Nakhodka.

Particularly alarming to protectors of endangered species, Transneft plans to get the oil here by building the pipeline through one nature preserve and along the southern border of a second preserve.

"If the pipeline is to be built in this area, a tremendous part of the tiny leopard and tiger habitat will be cut off," Dimitri Pikunov, a Russian biologist, said. "If a port is built near this reserve, no animals will stay. And this area represents the leopard's last stand."

The oil terminal project also presents a test between Japan's energy anxieties and its environmental concerns. Japan's development bank could finance up to 80 percent of the project, which would make it the largest ever loan by the bank. From an original price of $6 billion, costs have ballooned over the past two years, inflated by rising steel prices and the technical challenge of building in soils affected by differing conditions of permafrost.

Although Japanese studies say there will be enough oil found near the pipeline route to fill the pipeline, skeptics say the line could be a 21st-century Trans-Siberian Railway - a wonderful exercise in nation-building that has never made a profit.

Initially, the pipeline was to be far shorter, going to Skovorodino, a Siberian town 60 kilometers north of the Chinese border, and then angling south into northern China. But Japan overpowered China in the bidding war, although to appease Beijing, Russia has promised to increase its annual shipments of oil to China by rail to 300,000 barrels a day next year.

Moving aggressively, Japan offered $7 billion in assistance for construction of the line and billions more to help Japanese oil companies search for and develop oil in Eastern Siberia.

Tokyo lobbied heavily to get the entire pipeline - 1.6 million barrels a day - to the Sea of Japan. Although oil supply guarantees have not been worked out, Japan is expected to be the primary buyer. But much of the oil is expected to go onto the open market, available for shipping to South Korea, China or even the United States.