Paris, France (CNN) -- Three French hostages kidnapped in Nigeria have been released, the French government said Wednesday.
French Foreign Minister Bernard Kouchner announced their release "with great joy and deep relief," saying the Nigerian authorities had told him the news.
The three French nationals were taken hostage in Nigeria September 22 while on a boat belonging to the oil services company Bourbon.
They all appear to be in good health "despite difficult conditions while in captivity," Bourbon said Wednesday.
They are undergoing medical and psychological tests under the company's care in Nigeria before returning to France, the company said.
Kouchner expressed his "gratitude to the Nigerian authorities for the constant mobilization that led to the release of these three hostages."
They were captured in an attack on their ship, the Bourbon Alexandre, Bourbon said at the time.
It's not clear who captured them, but rebels said the following day that they had located them.
"The Movement for the Emancipation of the Niger Delta can confirm that it has located the three abducted French nationals and another individual abducted in a separate incident on the same night," the group said in a statement September 23.
"We are in negotiations with the abductors towards effecting a transfer of the men to the custody of MEND."
The statement by the group's spokesman Jomo Gbono did not give details on who the kidnappers were.
MEND did not issue a statement Wednesday when the French nationals were released.
It is not clear who the fourth hostage was.
Sixteen crew members were on the Alexandre when it was attacked by speedboats, Bourbon said. Thirteen remained on the ship and were unhurt, the company said.
The company provides ship and underwater services to oil and gas clients worldwide.
MEND, the key militant group in the oil-rich Niger Delta, is an umbrella organization of several rebel groups. It has been battling the government for years over a more even distribution of the country's oil wealth.
CNN's Christian Purefoy in Lagos, Nigeria, contributed to this report.