Radiation levels rose slightly in Russia's Far East on Tuesday but stayed within normal levels, officials said, as Japan struggled to cope with the worst nuclear accident since the 1986 Chernobyl disaster.
Radiation in Vladivostok, some 800 km (500 miles) northwest of Japan's Fukushima nuclear plant, were 1 microroentgen per hour higher at 0400 GMT than six hours previously, the regional interior ministry said.
Russia's military said it was on alert to evacuate people if required from Russia's Sakhalin island and the Southern Kuril island chain which is at the heart of a territorial dispute with Japan, Interfax news agency reported.
Called the Northern Territories by Japan, they are inhabited by Russians and one of the islands, Tanfilyeva, is a mere 6 km (4 miles) from Japan's coast.
Japan warned radiation levels had become "significantly" higher around a quake-stricken nuclear power plant after explosions at two reactors.
Japanese Prime Minister Naoto Kan urged people within 30 km (18 miles) of the facility north of Tokyo to remain indoors and conserve power, underscoring the dramatic escalation of Japan's nuclear crisis, the world's most serious since the Chernobyl disaster in Soviet Ukraine in 1986.