At least 421 bodies were buried in the grave in Maluku district in March, including about 300 stillborn babies and foetuses "abandoned in rivers, streams and even hospitals," the government said.
The EU office in the Democratic Republic of Congo called yesterday for an "urgent, transparent and credible" probe and offered to provide support "with all means at our disposal".
The United Nations Human Rights Office in Geneva also pushed for a probe to be completed in a "transparent, credible and independent manner."
The UN body "was also conducting our own independent human rights investigation, including by carrying out site visits and interviewing witnesses and family members of victims."
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Former colonial ruler Belgium and rights groups including Human Rights Watch backed up the call for an independent inquiry.
Kinshasa's interim governor Luzolanu Mavema has said the government had "absolutely nothing to hide".
The governor said he wanted to dispel rumours that the common grave could hold the bodies of government opponents who were killed during protests and mass arrests in January.