A North Korean man entered the US consulate in Vladivostok in the Russian far east this week to seek political asylum, a South Korean official said on Saturday.
A US embassy spokesman in Moscow confirmed the arrival of a man claiming to be a North Korean citizen on Thursday morning.
"We are working with the proper authorities to determine his status," he said. "Our standing policy is not to comment on whether an individual has applied for asylum."
It was not clear where the man wanted to travel to, the South Korean government official said. "Our government is asking the United States to respect his wish and to allow him passage to the destination of choice."
Japanese media said on Saturday that the man, thought to be aged around 40, had visited a local newspaper about a week ago to seek help. "I don't want to return to North Korea. Please help," the Mainichi Shimbun newspaper quoted him as saying.
Thousands of North Koreans are believed to be working in Russia's far east in the lumber and construction industries, another South Korean official said.
The incident is a rare case among the wave of North Koreans fleeing their country's communist regime in recent years. Refugees have tended to enter China and break into embassies and consulates seeking to go to the wealthier South.
There has been a rush of larger groups of North Koreans entering foreign missions in Beijing in recent weeks.
China on Tuesday condemned the spate of North Korean asylum bids, saying they undermined security.
A South Korean activist group said Chinese authorities arrested a group of 65 North Koreans on Tuesday, signalling a harder response by Beijing aimed at curbing the embarrassing number of North Korean break-ins.
The South Korean official said Seoul's policy of accepting North Koreans wishing to come to the South was unchanged.