British YouTube travel blogger Benjamin Rich, who specialises in remote and sometimes dangerous parts of the world, has been arrested at the Baikonur Cosmodrome in Kazakhstan, the head of the Russian space agency Roscosmos said on Saturday.
Dmitry Rogozin said in an online post that Rich - whose Bald and Bankrupt channel has 3.53 million followers - and Alina Tseliupa had been detained near one of the launch pads at Baikonur, which Russia rents from Kazakhstan.
Local authorities were determining "the exact level of participation in illegal activities" by the pair, said Rogozin, who posted photos of Rich's visa and Tseliupa's passport.
Some of Rich's videos feature a woman called Alina but it was not immediately clear whether she and Tseliupa were the same person.
The most recent video on Rich's channel was filmed in Syria and posted on April 24. In an Instagram post last week, Rich said "Syrian suntan and back in a country with Soviet mosaics" but did not specify where he was.
In London, Britain's foreign ministry did not have an immediate comment.
Baikonur, once a closed Soviet city, is now open to tourists who apply for permission from Roscosmos. It lies in the steppe around 1,100 km (680 miles) southwest of the Kazakh capital Nur-Sultan.