Moscow ? Russian authorities have detained an American and a British citizen in the far east after the pair made a rare crossing by foot of a frozen 56-mile stretch of the Bering Strait, officials said Tuesday.

The two were arrested in Chukotka province for failing to register with the authorities, Andrei Orlov, spokesman for the Federal Security Service’s northeastern border guard division, said on NTV television. All visitors are required to register with police within three days of their arrival in Russia, even if they have visas.

One of the adventurers, Briton Karl Bushby, said on his Web site that he and Dimitry Kieffer of Anchorage, Alaska, reached Chukotka province Friday. It took them 15 days to walk the 56 miles from Alaska to Russia.

The Web site said Bushby, a 36-year-old former paratrooper, made the crossing as part of a round-the-world walk that began in 1998 at the southern tip of South America.

The pair did not enter the country at a border crossing, so they had no stamps in their passports.