ZURICH (Reuters) - Switzerland will return to Nigeria around $321 million in assets seized from the family of former military ruler Sani Abacha via a deal signed with the World Bank on Monday, the Swiss government said. Transparency International, a corruption watchdog, has accused Abacha of stealing up to $5 billion of public money during the five years he ran the oil-rich country, from 1993 until his death in 1998. In 2014, Nigeria and the Abacha family reached an agreement for the West African country to get back the funds, which had been frozen, in return for dropping a complaint against the former military ruler s son, Abba Abacha. The son was charged by a Swiss court with money-laundering, fraud and forgery in April 2005, after being extradited from Germany, and later spent 561 days in custody.  In 2006, Luxembourg ordered that funds held by the younger Abacha be frozen. Now Switzerland, Nigeria and the World Bank have agreed the funds will be repatriated via a project supported and overseen by the World Bank, the Swiss government said.  The project will strengthen social security for the poorest sections of the Nigerian population. The agreement also regulates the disbursement of restituted funds in tranches and sets out concrete measures to be taken in the event of misuse or corruption,  it added. 