The Democratic Republic of Congo has banned some telecommunications executives from leaving the country after they resisted paying a new tax on the industry, according to people familiar with the matter.

Executives from the country’s four major carriers — Orange, Airtel Africa, Vodacom Group and Africell Holding — have been prevented from travelling and some have had their passports taken, the people said, asking not to be identified because they aren’t authorised to talk about the travel bans.

The government decree increases the tax on carriers’ revenue to 35% from 20%, one of the people said. The carriers initially rejected the new levy as “irregular and therefore unenforceable”, the Federation des Entreprises du Congo (FEC), the country’s main business association, said in June after Bloomberg’s initial report.

Orange’s local CEO and Africell’s DRC finance chief were blocked from leaving the country and had their passports confiscated at the Kinshasa airport earlier this month, according to a letter from the FEC to the interior minister. A Vodacom executive had her passport taken when she returned from a trip, one of the people said.

“To this day, no reason has been offered to the executives that justified these actions,” the letter said. The business group said it was “worried” and “strongly concerned by this measure affecting company directors, notably, those of the telecommunications sector”.

Representatives for Orange, Vodacom, Airtel and Africell declined to comment.

Read: Trouble for Vodacom and other operators in the DRC

FEC head Kimona Bononge said the group is engaging with the “highest authorities of the government on this subject”, and declined to comment further. Representatives for the government and telecoms regulator didn’t immediately respond to requests for comment.

The government is looking for ways to shore up its finances after years of mismanagement left the country without the revenue it needs. President Felix Tshisekedi’s government wants to broaden its tax base and boost its revenue as it undertakes a nationwide infrastructure development plan before elections scheduled for the end of next year.