DAR ES SALAAM – Tanzania said on Saturday it planned to nationalise diamonds whose value it put at $29.5-million belonging to a mine majority owned by London-listed Petra Diamonds after it accused the miner of under-declaring its mineral exports.

The diamonds were seized at the main airport in Tanzania's commercial capital Dar es Salaam on August 31 as they were being exported by Williamson Diamonds to Antwerp, Belgium.

The government first confirmed the confiscation on Thursday, when President John Magufuli was receiving audits by parliamentary committees on the mining sector.

"While Williamson Diamonds declared in its documentation that the value of the diamonds was $14.798-million, a fresh valuation done by the government established that the actual value of the diamonds is $29.5-million," Tanzania's Finance and Planning Ministry said in a statement.

"Among the legal action to be taken include the nationalisation of all the diamonds seized after it was established that there was cheating involved in declaring the actual value of the minerals," it said.

Petra owns 75% of the stake in Williamson Diamonds, with the remaining shares held by the government. The Williamson mine accounted for 18% of Petra's revenue last year.

Officials at Petra were not reachable for comment on Saturday. On Thursday, the company said its business was conducted in a transparent manner and that it had complied with all legislation in Tanzania.

The government halted the export of the diamond shipment and ordered a fresh sorting and valuation of the minerals to verify if the actual value was declared and all relevant taxes were paid.

The statement said Tanzania's Finance and Planning Minister, Philip Mpango, had also ordered a criminal investigation to be launched against all officials involved in declaring the value of the diamonds and issuing export permits.

The minister also ordered the arrest of employees of the diamond mine and public officials involved in the shipment.

"Tanzania is likely losing more than $46-million each year from the export of under-cleared diamonds through this airport," Mpango said in the statement.

Magufuli on Thursday ordered a review of Petra Diamonds' contract and asked senior public officials to resign over the outcome of an investigation into the mining sector.

One minister and a deputy minister quit their posts as requested, state-run television reported.

The president's actions are a continuation of a crackdown in a mining sector that accounts for about 4% of Tanzania's gross domestic product.

A parliamentary committee said on Thursday there were "gross irregularities" in the manner in which the Tanzanian government diluted its shareholding in Williamson from an initial 50% to the current 25%.

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