More than 40 people have been arrested in Burundi since the penalties for black market trading were increased last month, the ministry of public security has said.

The central African country has been short of foreign currency since foreign aid was frozen in 2016, after President Pierre Nkurunziza ran for a third term despite protests from opponents who said he was violating the terms of a deal that ended a civil war.

A ministry spokesman said that those who had been arrested were accused of "breaching the central bank regulation on foreign exchange".

The dollar fetches about 2,900 Burundian francs on the streets of the capital Bujumbura, nearly double the official rate of 1,876, traders said.

The central bank said in October that official reserves in the first quarter of the year covered only three weeks of imports, and has not answered requests for more recent information.

Dollars are sold at the official rate only to importers of essential goods such as fuel and fertiliser. Many businesses say they are unable to import merchandise and could be forced to shut down.