Anti-corruption groups in Nigeria are begging 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.

Adetokunbo Mumuni, director of the Socio-Economic Rights and Accountability Project, says the money is believed to be in banks or their offshore holdings from the United States and Britain, Switzerland and other European countries.

Mumuni and other activists met with Kerry on Wednesday in Abuja, Nigeria's capital.

Mumuni says Kerry told them asset recovery is a lengthy and complicated process but the U.S. government has lawyers and accountants working on it.

Mumuni says $6 billion has been recovered abroad from the loot of deceased military dictator Sani Abacha, but only $2.4 billion has been returned. Many more billions remain unaccounted for.