The ratings agency Fitch has downgraded Saudi Arabia's sovereign credit rating.

In a release Tuesday, Fitch lowered the rating to AA- from AA and maintained its "negative" outlook on the major oil producer.

Fitch said the downgrade was driven by the assumption that oil prices would stay near $35 a barrel this year and $45 a barrel in 2017.

This would have "major negative implications for Saudi Arabia's fiscal and external balances," Fitch said.

The rating downgrade makes it more expensive for sovereign issuers to borrow money.

Fitch noted that Saudi Arabia's deficit ballooned to 14.8% of gross domestic product last year from 2.3% in 2014, with the catalyst being the crash in oil prices.

It projected that the so-called deficit-to-GDP ratio would shrink only a bit this year and a little more in 2017 if oil prices recover.

Fitch said Saudi Arabia's efforts to raise money, including seeking a $10 billion loan from European banks, and its first Eurobond issue later this year, should help push the debt-to-GDP ratio back up to as much as 9.4% in 2017.

In February, S&P cut its rating to A-/A-2 from A+/A-1. In March, Moody's put its AA3 rating under review for a downgrade.

Here's the full downgrade release from Fitch:

Fitch Ratings has downgraded Saudi Arabia's Long-term foreign and local currency Issuer Default Ratings (IDRs) to 'AA-' from 'AA'. The Outlooks on the Long-term IDRs remain Negative. The Country Ceiling is affirmed at 'AA+' and the Short-term foreign-currency IDR is affirmed at 'F1+'.