It was carrying 12 passengers, six of them were rescued, while rescue teams were still searching for the two others, Lagos State Police spokesperson Patricia Amadin said.
The helicopter plunged into the waters of Lagos Lagoon near the Third Mainland Bridge which connects Lagos Island with the mainland.
Ibrahim Farinloye, spokesman for the National Emergency Management Agency in Lagos state, told Xinhua that the incident occurred around 15:00 local time and four people on board were killed.
"We don`t know exactly what happened but we have called the Nigerian Mission Control Center to confirm the distress signal," he said.
The Nigerian Civil Aviation Authority (NCAA) has also confirmed the crash.
A statement issued by the NCAA spokesman, Fan Ndubuoke, said the Bristow helicopter with registration number 5N-BDG-760540 was en route from one of the country`s oil rigs to Lagos.
The helicopter was scheduled to arrive in Lagos at 15:35 local time, while the number of casualties was yet to be determined, the statement said.
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