A major human rights watchdog left the Kimberley Process Monday, accusing the international diamond regulatory group of refusing to address links between diamonds, violence and tyranny.

Global Witness's departure raises questions about whether consumers can be sure the diamonds they buy aren't fuelling conflict.

In a statement, Global Witness cited what it called Kimberley Process failures in Ivory Coast, Venezuela and Zimbabwe.

"Consumers have a right to know what they're buying, and what was done to obtain it," said Charmian Gooch, a Global Witness founding director. "The diamond industry must finally take responsibility for its supply chains and prove that the stones it sells are clean."

The diamond industry, rights groups and 75 countries have worked together as members of the Kimberley Process since 2003 to impose requirements on its members to enable them to certify rough diamonds as "conflict-free" so that purchasers can be confident they are not funding violence. The project was born after wars in Sierra Leone and Liberia that were fuelled by "blood diamonds."

Partnership-Africa Canada, the other major rights group in the process, said it shared the frustrations of Global Witness, but would remain in the regulatory group in hopes of helping it reform.

Alan Martin, Partnership-Africa Canada's research director, said in a year, his group would look again at whether to remain.

"The Kimberley Process is a necessary but insufficient tool," Martin told The Associated Press, adding his group also would be working in other areas, including supporting legislation in Europe and North American that could make diamond  producers more accountable.

Eli Izhakoff, president of the World Diamond Council, the industry representative on the Kimberley Process, called Global Witness's departure "regrettable and unnecessary."

Izhakoff said the system was imperfect, but "you cannot contribute to the process if you are no longer engaged."

Global Witness 'a strong voice'

The departure of Global Witness would lessen the credibility of the Kimberley Process, said Farai Maguwu, head of Zimbabwe's independent Centre for Research and Development, which has monitored rights violations in that nation's diamond trade.

Global Witness was "a strong voice for communities living in diamond producing areas," Maguwu said. "Independent groups like Global Witness are important to ensure rights issues are upheld."

Last month, in a decision Global Witness called "disappointing" at the time, the Kimberley Process agreed to let Zimbabwe trade some $2 billion in diamonds from fields where human rights groups say miners have been tortured. Zimbabwe has denied allegations of human rights abuses in the fields.

Human Rights Watch has accused Zimbabwean troops of killing more than 200 people, raping women and forcing children to search for the gems in the fields.

"Over the last decade, elections in Zimbabwe have been associated with the brutal intimidation of voters. Orchestrating this kind of violence costs a lot of money," Global Witness's Gooch said Monday. "The Kimberley Process's refusal to confront this reality is an outrage."

Zimbabwe's Minister of Mines Obert Mpofu may have had such criticism in mind when he welcomed Global Witness's withdrawal on Monday.

He said the group had worked "to keep Zimbabwe from trading in diamonds. Now that Zimbabwe has been given the go-ahead they no longer have anything else to do."