Russian border guards are holding an American man who came ashore in the remote far eastern Chukotka region after sailing in a dinghy from Alaska.
The man’s motives are not clear. He was found exhausted near Lavrentiya, a coastal village, Russian media report.
At its closest point mainland Alaska is just 89km (55 miles) from Chukotka.
US diplomats are trying to resolve the case with Russian officials. Tass news agency said the man turned up on Wednesday and did not ask for asylum.
According to a source close to the investigation, quoted by Interfax news agency, the American “said he had been fishing along the Alaskan coast and had decided to sail from the US to China, but ended up in Russia”.
He is now being held in Anadyr, Chukotka’s main town.
Chukotka’s proximity to Alaska made it a highly sensitive region during the Cold War.
It is Russia’s second-least populated region, with about 50,500 inhabitants, and its infrastructure is poor. Many rural inhabitants still rely on traditional hunting and reindeer-herding.