The Sahel region sees a tenfold increase in the number of deaths in militant attacks while France urges other countries to do more.
The crew were seized after Hong Kong-flagged crude supertanker, the Nave Constellation, was attacked 77 nautical miles off Bonny Island on December 3.
The once-peaceful African country is at the centre of a gold rush and militant attacks.
The former French colony in the Sahel region of Africa, Niger, is looking to rid its national anthem of any remnants of French colonisation.
French Defence Minister Florence Parly said an investigation has been opened to determine the exact circumstances of the accident.
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The militant attack on a police base in the north of the country also left one police officer dead and seven officers injured, police said.
The Sahel region is witnessing rising malnutrition levels, an internally displaced population and widespread extremist violence.
This is the latest in a long chain of tit-for-tat violence that has plagued Nigeria's northwest.
The move is thought to be in retaliation for Nigeria shutting its borders. The country made the protectionist move to stop cheap imports damaging local markets.
The army says 24 fighters were killed in the first operation on Friday and a further eight in a second on Saturday, in the north of the country.
The Independent Electoral Commission of Nigeria says over 31,000 police had been deployed, as well as 87 gunboats, to prevent, or deal with, any electoral violence.
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