Nigeria's aviation regulatory agency says it is suspending operation of all Sikorsky model S-76C++ helicopters following two crashes in six months.

One helicopter crash-landed off an oil rig in the Atlantic Ocean on Wednesday. Nine passengers and two crew members were rescued from the sea.

Another Sikorsky nose-dived into a lagoon on Aug. 10 in Lagos, the commercial capital, killing six of the 12 people on board including the co-pilot.

The U.S.-made aircraft are operated in Nigeria by Bristow, a Houston, Texas-based company that provides air services to offshore oil and gas companies.

Director general Capt. Muhtar Usman said Thursday the Nigerian Civil Aviation Authority has "decided to suspend the operation of the Bristow Helicopters Sikorsky S-76C++ type till further notice" pending "a full audit."

Bristow said it has complied.