The Niger Delta Avengers group has slashed Nigeria's oil production by 600,000 barrels per day.
Some Nigerians are wondering why he doesn't receive treatment at home.
The Nigerian army says six people were killed in a separate attack in the Niger Delta.
The oil-producing region has been the site of multiple attacks by a militant group in recent months.
Militants including the Niger Delta Avengers have carried out attacks on oil pipelines, cutting production.
A new militant group called the Niger Delta Avengers have been attacking oil pipelines in recent weeks.