JOHANNESBURG — An advocacy organization that helped to establish an international certification program to prevent the sale of so-called blood diamonds withdrew from the coalition on Monday, saying the effort was no longer effective.

The organization, Global Witness, is the first advocacy group to leave the program, known as the Kimberley Process, which was set up in 2003 because conflicts in Angola and Sierra Leone were being fueled by diamond sales. Global Witness had expressed concerns about how the Kimberley Process was operating for some time; it said the final straw was the decision last month to allow Zimbabwe to export diamonds from the Marange fields, where there have been reports of widespread human rights abuses by government security forces.

“It’s the most egregious situation that we’ve seen since the Kimberley Process was launched, where diamonds have been fueling violence and human rights violations,” said Annie Dunnebacke, the senior campaigner for Global Witness, “and the Kimberley has really failed to deal with that effectively.”

More than 70 countries, including the United States, have signed the Kimberley Process agreement to adhere to United Nations-backed safeguards on trade in rough diamonds.