At least $2 billion of diamonds from Zimbabwe's Marange fields have been stolen by people linked to President Robert Mugabe's party, a diamond watchdog has said, although the state mining firm said the gems were sold transparently.

Partnership Africa Canada, a member of the Kimberley Process initiative against "blood diamonds", said in its latest report that Marange diamonds have only benefited a well-connected elite and that the military had also been given mining concessions.

The Kimberley Process has certified Marange gems, but human rights groups have raised concerns about potential abuses at mines and Mugabe's opponents fear proceeds from diamond sales will be used to fund a war chest for elections expected in 2013.