The sister of Nigerian Petroleum Minister Diezani Alison-Madueke has been kidnapped in the latest case of abductions targeting political families in the oil-rich African nation.

Osio Agama was seized at gunpoint in the oil hub of Port Harcourt on Tuesday night, Rivers state police spokesman Ahmad Muhammad said on Friday.

The Nigerian police "were not aware if any ransom demand had been made" and the motive for the abduction was unknown, Muhammad said.

In recent years, the southern oil-producing Niger Delta region, home to Africa's largest oil industry, has witnessed waves of ransom kidnappings.

Some observers have attributed political motives to attacks against Nigeria's powerful families.

Earlier in 2104, the uncle of President Goodluck Jonathan was also kidnapped in Bayelsa state in the Niger Delta.

In December 2012, Finance Minister Ngozi Okonjo-Iweala's mother was abducted in Delta state and released a week later.

Okonjo-Iweala said that the abduction was linked to the Finance Ministry’s crackdown on oil companies abusing the country's fuel subsidy plan, but her claim has never been corroborated.

Okonjo-Iweala and Alison-Madueke are considered the most influential members of Jonathan's cabinet.

Despite daily production of two million barrels of oil, the area suffers abject poverty and high unemployment rate and witnesses rampant gang activities and abductions which mostly target prominent politicians, businesses and foreigners.