A row has broken out over reports that Zimbabwe has been allowed to sell diamonds from its rich Marange mines.
A statement by the current chair of the Kimberley Process - the industry's main certification system - appears to allow exports to resume.
However, the US, the EU and other groups signed up to the process insist that no consensus had been reached at a meeting in Kinshasa.
All Kimberley decisions are supposed to be consensus-based.
The Kimberley Process (KP) is an international initiative set up to stem the flow of "blood diamonds" - rough diamonds used by rebel movements to finance wars against legitimate governments.
All diamond exports by Zimbabwe were suspended in 2009 after allegations of widespread abuses and killings by the security forces at the Marange mines.
Following the Kimberley meeting in Kinshasa, Mathieu Yamba of the Democratic Republic of Congo, which holds Kimberley's rotating chair, issued a statement that said exports from Marange could go ahead.
He later issued an explanatory note saying he had approved what he called "a compromise document stating the essential views expressed by each and everyone".
"Thus, on the basis of this document, Zimbabwe will continue exports," he said.
He told AFP news agency: "We have decided to lift the measure which prevented Zimbabwe from exporting its diamonds in the Mbada and Kanadai mines in the Marange region."
African countries, China and India are reported to have supported the decision but it was opposed by Western nations, human rights groups and industry.
Zimbabwe Mines Minister Obert Mpofu told the state-run Herald newspaper in Harare that "a breakthrough" had been achieved.
"We want to be treated like any other country. I'm going to sell our diamonds now," he told Reuters news agency.
Outrage at the decision prompted representatives of rights groups and resources watchdogs to walk out of the meeting before the final statement. They said the values of the organisation were being undermined.
EU foreign policy chief Catherine Ashton said in a statement that "the text circulated by the KP chair was not agreed by consensus and is not therefore valid under KP rules and procedures".
US state department spokesman Victoria Nuland said that Washington was "deeply disappointed" by the outcome of the Kinshasa meeting.
"Contrary to some reporting, the Kinshasa Intersessional did not reach a consensus text. The chair has circulated a text to participants which did not attract consensus," she said in a statement.
"We believe that work toward a solution must continue, and that until consensus is reached, exports from Marange should not proceed."
UK Minister for Africa Henry Bellingham said Britain was "deeply concerned" by the Kimberley statement.
US-based diamond group Rapaport Trade quickly advised its members not to trade in Marange gems.
"Marange goods (are) expected to be released shortly," Rapaport said in an advisory to members.
"Responsible buyers should require supplier guarantee that they are not selling these diamonds to them."
The Marange fields have been called the richest diamond find of the decade.
Human rights groups say local people have been forced to work in the diamond fields, smuggling is rife and some mines in Marange are controlled by Zimbabwe's military, who channel some of the funds to President Robert Mugabe's Zanu-PF party.
The Zimbabwean authorities deny the accusations.