"The transfer became possible with the signing of an agreement with the World Bank to monitor Nigeria's use of the funds," said Livio Zanolari, a spokesman for the Swiss Justice Ministry.
The $290m was to be paid immediately to the Bank for International Settlements in Basel, Mr Zanolari said. The international bank will then credit it to the Central Bank of Nigeria.
A second instalment of $170m will be made when assets have been converted to cash, said Mr Zanolari, but he said he did not know how long that would take. That second instalment will complete the payment of all the funds found in Switzerland, he said.
The money has been frozen in Swiss banks since 1999. The Swiss supreme court gave a green light for the return last February when it rejected an appeal by the Abacha family.
But the Swiss cabinet said it wanted the World Bank's involvement organised first to guarantee the money would go toward development projects in areas such as health, education and infrastructure, as promised by Nigeria.
The agreement with the World Bank has now been signed, Mr Zanolari said.
Olusegun Obasanjo, the Nigarian President, accused Switzerland in May last year of stalling on the return of the funds, but Swiss officials said it was held up only because of the need to work out details with the World Bank.
The Nigerian government had accused Abacha of creating a criminal organisation after his takeover in 1993 and plundering $2.2bn from state funds until his death of an apparent heart attack in 1998.
After he died, Swiss officials blocked about $730m in bank accounts in the country linked to Abacha and his associates. Some $216m has previously been returned to Nigeria.
The Nigerian government has been pushing, with some success, for the return of Abacha's looted funds from other countries, including Britain, Luxembourg, Lichtenstein and Austria. Transparency International, the anti-corruption organisation, ranks Nigeria, the world's seventh largest exporter of oil, as the world's third-most corrupt country.Reuse content