A helicopter operated by Nigeria's navy crashed in the southern part of the country on Saturday, killing a state governor, a former national security advisor and four others, officials said.
A statement from President Goodluck Jonathan said the governor of the northern state of Kaduna, Patrick Ibrahim Yakowa, and ex-national security advisor Owoye Azazi were both killed along with two aides and the two naval officers operating the helicopter.
"President Jonathan has ordered an investigation into the cause(s) of the crash," the statement said.
The navy said the helicopter was headed to the oil hub of Port Harcourt and went down in the Nembe area of oil-rich Bayelsa state at about 3:30 pm (1430 GMT).
Emergency workers who deployed to the scene have completed the search-and-rescue operation, the National Emergency Management Agency said.
The operation "led to the recovery of the bodies of the six passengers on board the helicopter," NEMA said, adding: "The bodies have been deposited in the mortuary."
Jonathan expressed "utter shock and sadness" over the crash that killed the governor of the embattled state along with a retired general and former security aide who was considered one of his top political allies.
Religiously divided Kaduna has seen a series of clashes between Christian and Muslim residents that have left scores dead this year.
The state has also been among the hardest hit by radical Islamist group Boko Haram, which has repeatedly attacked Christian churches with suicide blasts in and around the state capital Kaduna City.
The fallen governor "demonstrated an unyielding capacity in wielding together varying fragile interests," the ruling Peoples Democratic Party said in a statement.
"Even in the face of daunting security challenges, nothing came close to compromising his iron-cast resolve," the party's condolence message further said.
It was not clear how Yakowa's death would affect the precarious security situation in Kaduna.
Azazi, a prominent figure in the both political and military circles, also played a role in trying to manage the Boko Haram insurgency, which has killed hundreds in Nigeria since 2009.
Jonathan sacked him in January, following the group's deadliest ever attack, which claimed at least 185 lives in the northern city of Kano.
Like Jonathan, he was a native of Bayelsa, where he was seen as having huge political influence.
The two officials were said to be travelling to a funeral in Port Harcourt.
In October, a small plane piloted by Taraba state Governor Danbaba Suntai crashed in northeastern Nigeria.