FOR most of the 20th century, Russia's Far East has been off limits to Americans. The closest part of Russia to the United States, Russia east of Siberia, seemed also the most forbidding. Among its sites: Magadan, port of entry to Stalin's Arctic gold mines; Sakhalin Island, home base to the Soviet fighter that shot down Korean Air Lines Flight 007; and Vladivostok, a city closed to all Westerners when it was home port to the Soviet Pacific fleet.

But the ice curtain has melted, and Alaska Airlines is building a network of flights between Seattle and Anchorage and Russia's back door.

It began flying there in 1991. In its latest expansion, it will offer weekly service starting on May 10 from Seattle and Anchorage to the airline's fifth destination in the region, Yuzhno-Sakhalinsk, the provincial capital of Sakhalin Island.

In another innovation, this summer Americans holding multiple-entry visas will be able to city-hop within Russia on Alaska Airlines.