Sunday Aghaeze, Nigeria State House via Associated Press
In this photo released by the Nigeria State House, U.S. Secretary of State John Kerry, left, talks to Nigeria's President Muhammadu Buhari, before their bilateral talks at the State House in Abuja, Nigeria, Tuesday Aug. 23, 2016. Kerry arrived Tuesday in Nigeria to hold talks with leaders of the West African nation that are expected to focus on regional security and extremism.

LAGOS, Nigeria — Anti-corruption groups in Nigeria have begged U.S. Secretary of State John Kerry to help speed the return of billions of dollars looted from the country's treasury by local officials.

Kerry met Wednesday with Adetokunbo Mumuni, director of the Socio-Economic Rights and Accountability Project, and other activists before ending his latest Africa visit.

Mumuni said the looted money is believed to be in banks or their offshore holdings from the United States and Britain, Switzerland and other European countries.

Kerry told the group that asset recovery is a lengthy, complicated process but the U.S. government has lawyers and accountants working on it, Mumuni said.

The United States in 2014 froze nearly half a billion dollars of illicit funds from the loot of deceased military dictator Sani Abacha but has yet to say how and when the money will be repatriated.

President Muhammdu Buhari, who won 2015 elections on a promise to halt graft, has estimated that $150 billion has been stolen by government officials over the past decade.

Dozens of current and former officials have been detained since Buhari took office, but Mumuni said court cases are being delayed by a corrupt justice system.

On Tuesday, Kerry linked corruption to Nigeria's ongoing Islamic uprising that has killed an estimated 20,000 people. The first leader of the homegrown extremist group Boko Haram became popular because of his sermons against corruption.

"There is nothing more demoralizing, more destructive, and more disempowering to any citizen than the belief that the system is designed to fail them," Kerry said.

Mumuni's organization has ongoing lawsuits to force the government to investigate, among other things, $20 billion in oil revenues that disappeared in 2014-2015 under the government of President Goodluck Jonathan and the diversion of $15 billion meant to buy arms to fight Boko Haram.