By Camillus Eboh

ABUJA (Reuters) - Kidnappers who held the mother of Nigeria's finance minister for five days told their captive that her daughter must resign because she was blocking fuel subsidy payments, the minister said on Monday.

Kamene Okonjo, the 83-year-old mother of the former World Bank director Ngozi Okonjo-Iweala, was abducted in southeastern Delta state on December 9 and freed following a large-scale military search.

Authorities did not say if the motive was political or for financial gain or whether they had captured the kidnappers.

Okonjo-Iweala has tried to root out corruption from a multi-billion dollar fuel subsidy scheme - creating many rich and powerful enemies in the process.

"The kidnappers spent much of the time harassing her. They told her that I must get on the radio and television and announce my resignation," Okonjo-Iweala told reporters in Abuja.

"When she asked why, they told her it was because I did not pay oil subsidy money," she added.

Kidnapping is rife in Africa's top oil producer, making millions of dollars a year for criminal gangs. It is common across the south, especially in the Niger Delta, and is almost always done for ransom.

Neither the authorities nor Okonjo-Iweala have said if a ransom was paid for her mother's release.

President Goodluck Jonathan, with the strong backing of Okonjo-Iweala, attempted to remove the fuel subsidies in January but was forced to partially reinstate them after more than a week of strikes brought the country to a standstill.

Several reports have since shown the subsidy scheme to be rife with corruption, with the state paying for twice as much fuel as the country consumes, a scam that one parliamentary probe said cost $6.8 billion over three years.

Okonjo-Iweala's ministry has subjected subsidy claims to more rigorous checks and forced oil marketers - including some of the richest and most powerful people in the country - to prove the fuel they claim for is being delivered.

Some marketers have been arrested by the financial watchdog.

"For marketers whose transactions are proven to be fraudulent ... we cannot and we will not pay. We will not back down on this," Okonjo-Iweala said on Monday.

The government is likely to spend over 1 trillion naira on the subsidies this year, more than 20 percent of the federal budget.

Nigeria's is among the world's top 10 crude oil exporters but due to decades of mismanagement and theft its refineries operate at under 30 percent capacity, forcing it to import 80 percent of its fuel needs.

Nigeria's army said last week that soldiers had arrested 63 people during the search for Okonjo-Iweala's mother. ($1 = 157.6500 Nigerian naira)

(Writing by Joe Brock; Editing by Robin Pomeroy)