The company last week acknowledged a report published by non-government organisation RAID alleging abuses related to security at the mine, which was managed by Williamson Diamond (Petra 75%; government 25%), a security contractor and local police.

RAID said its report, "The Deadly Cost of ‘Ethical' Diamonds", might understate the level of violence and said it had found evidence of at least seven killings and 41 assaults since Petra acquired Williamson in 2009, based on research spanning a year including 118 interviews with residents, community leaders, former security personnel and others.

Petra said it had been cooperating with RAID since becoming aware of the allegations on August 29 and had established a sub-committee to oversee the matter, with an investigation due by year-end.

It said Willamson's heads of security and general services had both been suspended, pending the outcome of the investigation, and an interim head of security had been appointed.

It also aimed to have a new security service provider contract in place in January, with the incumbent's contract due to expire in December.

Other measures included developing a comprehensive stakeholder engagement plan, opening a community office and launching a dedicated grievance desk and developing a new interactive radio programme.

UK-based law firm Leigh Day had also filed claims in the High Court in September on behalf of 32 individuals, alleging human rights violations by Petra and mine operator Williamson Diamonds.

After a failed sales process, Petra last month announced a debt for equity "restructuring solution" - to address a US$650 million note debt - which would give noteholders 91% of its enlarged share capital. 

Petra shares (LSE: PDL) closed up 1.3% on Friday to 1.53p, well below January's circa 11p, capitalising the company at £13.2 million (US$17.4 million).