Nigeria's Finance Minister said on Thursday she disagreed with what she called a "surprise" downgrade of the country's credit rating by Moody's, insisting the government was already addressing the agency's concerns.

Moody's downgraded the West African oil producer last week to Caa1 from B3, saying the government's fiscal and debt position was expected to keep deteriorating, an announcement that sent Nigeria's dollar-bond and currency forwards tumbling.

"Moody's downgrade came as a surprise to us because we had presented all the work that we have been doing to stablise the economy," the minister, Zainab Ahmed, told reporters in Abuja.

"But these are external rating agencies that don't have the full understanding of what is happening in our domestic environment."

She said she expected S&P's rating, due on Friday, would be more positive.

"S&P's assessment is not the same as Moody's. They have come out with a much better assessment," she said.

Nigeria has faced oil production shortages due to crude theft in recent years, though production has started to recover.

It has also suffered chronic dollar shortages coupled with high debt service which has eaten into government revenues.

Moody's cited these factors as reasons for its downgrade.