The Kimberley Process aims to stem the flow of blood diamonds
The Kimberley Process certification scheme, which aims to stop the use of diamonds to fund conflict, is failing, according to a campaign group.
Global Witness pointed to the smuggling of diamonds from Ivory Coast and an alleged massacre of diamond diggers by the military in Zimbabwe last year.
The rights group, which lobbied to set up the scheme in 2003, says it is not being adequately enforced.
Officials are meeting in Namibia to review the Kimberley Process.
Namibia is hosting the three-day conference in Windhoek because it currently heads the scheme.
Participants in the Kimberley Process are forced to certify the origin of any diamond being traded.
This seeks to assure consumers that by purchasing diamonds they are not financing war and human rights abuses.
But late last year reports emerged in Zimbabwe of a military-led slaughter of up to 150 miners in the eastern Marange diamond fields.
The World Federation of Diamond Bourses in April banned the sale of diamonds from Marange, but Kimberley did not.
From Windhoek, Annie Dunnebacke, a spokeswoman for London-based Global Witness, told the BBC's Network Africa programme: "This is not something the Kimberley Process can stand by and accept from one of its participants."
She also said statistical anomalies were being reported by signatories.
"According to the Kimberley Process, Guinea's [diamond] exports have increased by around 500% in the past two years, which obviously raises eyebrows and raises questions," she said.
The campaigner noted there was significant illicit cross-border movement of precious stones between Sierra Leone and Guinea.
She also said that in Ivory Coast - which is due to hold elections in November following a bloody civil war in 2002 that was partly funded by conflict diamonds - production of the stones appeared to be increasing and they continued to be smuggled out to reach legitimate markets, despite UN sanctions.
"The clock is running out on Kimberley Process credibility," Ms Dunnebacke added.
Bernhard Esau, the Namibian deputy minister of mines and current chairman of the Kimberley Process, said "gaps can be strengthened" over enforcing the scheme in Zimbabwe.
But he also insisted Kimberley had been "hands on with regard to the illicit diamonds from Marange".
The Kimberley Process emerged from global outrage over conflicts in countries like Liberia and Sierra Leone, largely funded by the plundering of diamond resources.