LITTLE DIOMEDE, ALASKA, AUG. 7 -- Endurance swimmer Lynne Cox spent two hours and five minutes plying the icy Bering Strait today, completing an unprecedented swim from American to Soviet territory.

Escorted by a Soviet vessel and two Eskimo walrus-skin boats carrying researchers and reporters, Cox arrived at the Soviet island of Big Diomede at 1 p.m. after a swim of four miles in 44-degree water.

Cox, 30, was welcomed to Big Diomede by about 30 Soviet journalists, sports officials and Siberian natives. They zipped her into a sleeping bag in a charcoal-heated tent to rewarm her body to a normal temperature, then celebrated her feat with a beach-side picnic.

"They were incredible. They met us at the date line with escort boats, and they had a tea party for us," Cox said in a telephone conversation with Alaska Gov. Steve Cowper (D) on her return here.

Cox, of Los Alamitos, Calif., is a pioneer of long-distance swims around the world.

The 100 residents of Little Diomede turned out this morning as Cox and an entourage of 11 researchers and reporters piled into two skin boats for the trip through fog and drizzle and calm seas to the starting point.

Adults cheered and children dropped red and blue balloons into the water.

She started her swim from Little Diomede's south end to compensate for tides and currents. Although the distance to Big Diomede is 2.7 miles, Cox had to swim four or more miles to get across.

A 20-foot Soviet vessel joined the escort around noon, when Cox reached the swim's halfway point at the international date line.

The two skin boats accompanying Cox were allowed to continue, but several other boats piloted by Little Diomede residents were turned back. Big Diomede, uninhabited except for a military reservation, is normally off-limits to Western travelers.

By the time Cox reached Big Diomede, her body temperature had fallen a few degrees, but she was not in danger of hypothermia, said Bill Keating, a British physiologist accompanying her. Cox said she felt tired during the last half-hour of the swim but was encouraged by the sight of the people on the beach.

"We were lucky we had pretty good conditions," she said. "The water was a little warmer than expected."

Cox spent the past several weeks in the northwestern Alaska town of Nome swimming in the 54-degree water of Norton Sound to prepare herself for what she expected to be a 2 1/2-hour ordeal. Physicians hoped her swim would help them to learn more about the body's ability to withstand cold.

"It's not just a question of physiology," Keating said. "It's a question of her making herself do the things that are needed to survive."

Doctors say the 5-foot, 6-inch Cox is able to cope with the cold because an estimated 40 percent of her 209-pound body weight is fat, which in effect serves as an insulating wet suit. Twenty-five percent body fat is considered the limit of the normal range for women.

Her aerobic capacity is estimated to be equal to that of a world-class marathon runner, enabling her to swim for long periods.

Cox has set several records for crossing the English Channel and has made pioneering swims in remote spots around the world. She began her swimming career in southern California.