(Adds Total confirmation, detail on Usan in paragraph seven)
LONDON, May 30 (Reuters) - A force majeure has been declared on the Usan grade of Nigerian crude oil, operator Total confirmed on Thursday, adding to the problems facing Africa's largest oil exporter.
A spokesman for Total in Geneva confirmed production was completely halted, but said there was no further information on the outage, or how long it was likely to last at this stage.
A force majeure exempts parties to a contract from their obligations due to circumstances beyond their control.
Four cargoes of Usan representing around 127,000 barrels per day of Usan were due to be exported in June.
The force majeure for Usan adds to those on Royal Dutch Shell's Bonny Light and Eni's Brass river.
Shell declared a force majeure on Bonny Light on March 5 because of "unprecedented" levels of theft.
Shell said on Thursday the force majeure was still in place.
Usan is an offshore field, and so is not subject to theft like Brass River or Bonny Light grades of crude oil.
Analysts say that the unreliability of supply is encouraging buyers to look elsewhere for more steady streams of oil. (Reporting by Simon Falush, additional reporting by Joe Brock in Abuja; editing by James Jukwey)