MOSCOW (REUTERS) - A gunman opened fire in a Russian Orthodox cathedral on the island of Sakhalin, off Russia's eastern coast, on Sunday, killing a nun and a churchgoer and wounding six other people, officials said.

The shooting happened at a time when Russian security forces are on high alert due to concerns about possible attacks on the Winter Olympics in Sochi, thousands of kilometres to the west. There was no apparent link to the Sochi Games.

The gunman was detained at the scene after the shooting at the main cathedral in the city of Yuzhno-Sakhalinsk, on the island in the Sea of Okhotsk, north of Japan, the federal Investigative Committee said.

The suspect worked as a security guard in Yuzhno-Sakhalinsk, it added.

There was no immediate word on a motive in the attack, which came days after a Moscow teenager killed a teacher and a policeman and held classmates hostage in the first major school shooting in Russia.

Most of the wounded in Sunday's mid-afternoon attack were shot in the legs and their lives were not in danger, RIA cited the regional leader of the Russian Orthodox Church, Archbishop Tikhon, as saying.

He said a prayer service for the victims would be held later on Sunday at the cathedral, which is the main Russian Orthodox church in a region that includes Sakhalin and the Kurile island chain. The part of the church where the shooting took place would have to be cleaned of victims' blood and blessed, he added.