Saipem, a subsidiary of Italian energy company Eni, said one of the three legs of its Perro Negro 6 drilling rig collapsed while operating near the mouth of the Congo River between DRC and Angola.
The company said one member of the 103-man crew was missing and six others suffered minor injuries. The rig later sank.
The company said Tuesday it dispatched an emergency response team to coordinate with the Angolan government.
"No environmental impacts have been reported, and all the prevention measures are being implemented," the company said.
Saipem said it had insurance coverage necessary to pay for the equipment loss, any possible environmental damage and the cost associated with wreck removal.
The company did not say what reserve area it was targeting with the rig. Angola is a member of the Organization of Petroleum Exporting Countries and ranks second behind Nigeria in terms of oil production in Sub-Saharan Africa.