Moody's Investors Service has cut its outlook for the debt ratings of Saudi Arabia and three other Gulf states while lowering Bahrain's rating to junk, citing concern over the impact of low oil prices on their finances.

Saudi Arabia's Aa3 rating was placed on review for a possible downgrade, Moody's said late on Friday, adding that it would study whether Riyadh's efforts to expand its non-oil revenues and diversify its economy were likely to work.