Tanzanian President John Magufuli said on Thursday he had ordered a review of a contract belonging to Petra Diamonds Ltd in Tanzania and asked senior public officials to resign over the outcome of a probe into the sector.

“I have endorsed all the recommendations of the parliamentary probe committees for the review of the Williamson diamond mine contract,” Magufuli said in a televised broadcast.

London-listed Petra has a 75 percent stake in the mine, while the government owns the rest. Shares in the company were down 1.6 percent in morning trade.

A spokeswoman for Petra was not immediately able to comment.

Magufuli said he had also ordered law enforcement agencies to investigate allegations of under-declared diamond exports.

Magufuli’s actions are a continuation of a crackdown in the mining sector.

He has sent shockwaves through the mining industry in Africa’s fourth-largest gold producer since his election late in 2015 with actions he says are aimed at ensuring that mining companies pay a fair share of taxes.

Tanzania has passed new laws to increase mining taxes, to force companies to renegotiate their contracts and to allow the state to own up to 50 percent of shares in mining companies.

In March, it banned exports of copper concentrate or mineral sand for processing abroad, which particularly hit gold miner Acacia Mining, the biggest in the country.