PORT HARCOURT, Nigeria (AFP) - The kidnapped sister of Nigeria's oil minister, who was abducted last week in the oil city of Port Harcourt, was freed in a joint operation by security forces, police said Wednesday.

"Osio Agama has been safely rescued during a joint operation by the police, military and SSS (State Security Service)," Rivers state police spokesman Ahmad Muhammad told AFP.

Muhammad said two suspects had been arrested over the Oct 21 abduction of Agama, whose sister Diezani Alison-Madueke leads Africa's largest oil industry.

Muhammad did not say if ransom was paid for the release, the latest targetting relations of prominent Nigerians in recent months.

One of the most prominent cases came in December 2012, when Finance Minister Ngozi Okonjo-Iweala's 82-year-old mother Kamene Okonjo was abducted from her home, in neighbouring Delta state.

Okonjo-Iweala and Alison-Madueke are widely seen as the two most powerful members of President Goodluck Jonathan's Cabinet.

Jonathan's 70-year-old uncle was also kidnapped earlier this year in Bayelsa state, also in the Niger Delta.

Some have sought to attach a political motive to attacks targeting Nigeria's most powerful families.