The oil-rich kingdom of Saudi Arabia was dealt another blow by the global oil glut on Tuesday, suffering its second credit ratings downgrade in recent months.

International ratings agency Fitch said in a memo that it was lowering the kingdom's "AA" credit rating to "AA-," warning that continual downgrades were more likely than a reversal of the trend due to the low price of oil.

Fitch said the downgrade was based on Saudi Arabia's decision to lower its average oil price assumptions for 2016 and 2017 to $35 and $45 a barrel.

The Saudi government's deficit grew to 14.8 percent of the country's gross domestic product last year, from 2.3 percent in 2014. Before that, the government saw continuous surpluses, Fitch pointed out.

The Fitch downgrade follows similar action taken by Standard & Poor's.