About 70 diamond producing and importing countries at a meeting in South Africa on Thursday passed a pact paving the way for individual countries to volunteer for review by their peers.
US diamond traders and a spokesman for South African diamond giant De Beers said the Democratic Republic of Congo (DRC) and the Congo Republic had become the first countries to volunteer for inspection.
The DRC produces nearly $400 million worth of diamonds a year while Congo Republic exports about $200 million.
The countries meeting in South Africa are members of the Kimberly Process Certification Scheme, aimed at stopping trade in "blood diamonds" that is blamed for fuelling war and instability in many African countries.
Earlier meetings of the group resulted in agreements to send review teams to countries where there was credible and significant evidence of trade in diamonds from conflict areas.
At its latest meeting, member-countries agreed to expand the monitoring system by calling on countries to volunteer for review visits to show their diamond industries are clean.
The agreement reached, however, did not specify sanctions for countries that do not volunteer for review, but officials said they believed the public pressure to open up for inspection would compel most to comply.
International watchdog groups had all along lobbied for tougher measures including a system of mandatory checks.
"It is not everything we wanted, but it is a step in the right direction," Alex Yearsley from the watchdog group Global Witness said.
"You will never be able to stop diamond smuggling completely," he said.