Nigeria's army said on Thursday soldiers had arrested 63 people in raids as they searched for the finance minister's 82-year-old mother, kidnapped from her home on Sunday.

It is not yet known whether the abduction of Kamene Okonjo, mother of former World Bank director Ngozi Okonjo-Iweala, was political or for financial gain.

The latest detentions were in oil-producing Delta state. "Yesterday the Four Brigade raided Ogwashi-Ukwu in search of Mama," army spokeswoman Roseline Managbe told Reuters.

"Those arrested are being questioned," she added.

Criminal gangs regularly kidnap people for ransom in Nigeria, but it is rare for them to target members of the political elite.

Two policemen were also arrested this week on suspicion of aiding the kidnappers of Okonjo.

Managbe said two Lebanese men working for Nigerian construction company Setraco had been abducted on Tuesday in Delta state by gunmen who killed a soldier assigned to protect them.

Gangs operate throughout the Niger Delta, home to Africa's largest oil industry. Most people abducted are Nigerians but foreign oil and construction workers have also been frequent targets.

The finance minister, a campaigner against corruption, has received threats in the past, her special adviser Paul Nwabuikwu said on Sunday.