Two senior executives at Binance have been detained in Nigeria as Africa’s most populous country tries to rein in speculation on the naira by cracking down on cryptocurrency exchanges.

The executives flew to Nigeria following the country’s decision to ban several cryptocurrency trading websites last week but they were detained by the office of the country’s national security adviser and their passports seized, according to people familiar with the matter. An adviser to the office did not immediately respond to a request for comment. Binance declined to comment.

Nigerian authorities have turned their focus to cryptocurrency websites after a rapid devaluation of the naira that helped drive inflation to an almost three-decade high of 29.9 per cent. The sites have emerged as an alternative for trading and establishing an unofficial price for the Nigerian currency. Following the detention of the executives, Binance halted trading of the naira against bitcoin and tether digital coins on its exchange.