Credit rating agency Standard & Poor's signalled on Friday that oil-exporting countries face fresh downgrades as crude prices fall further and that it could repeat last year's move when it made a big group of cuts all at once.

The plunge in oil prices since mid-2014 has brought a blizzard of downgrades, including for Russia and Brazil, which have been stripped of investment grade, number one producer Saudi Arabia, and Venezuela, where the oil rout has raised fears of a sovereign default. 

But a further 20 percent slump since the start of the year which few had foreseen could mean another batch of cuts is imminent, S&P's EMEA head of sovereign ratings Moritz Kraemer, told Reuters.

"This is not the run-of-the-mill commodities swing, this is something different altogether," Kraemer said.

"We had an important number of downgrades last year in Africa, the Middle East and the CIS, and if our outlooks continue to perform as an indicator of where the ratings may next go, more might be coming up this year."

S&P currently has Azerbaijan, Bahrain, Kazakhstan, Oman, Russia, and Saudi Arabia on negative outlook in its Europe, Middle East and Africa region, as well as Brazil and Venezuela in Latin America.

Nigeria, another oil-dependent emerging market giant, has a stable outlook, but pressure is mounting almost everywhere as governments see their revenues bleed away.

"There is a big overhang of negative outlooks and a rapidly growing negative overhang," Kraemer said. "A large number of them are commodity exporters, specifically oil exporters, so that tells you pretty strongly that the balance is tilting to downgrades."

Azerbaijan, one of the few big oil countries to escape a rating downgrade last year -- although its outlook was cut -- could potentially be the first to fall when its bottom-of-investment-grade BBB- rating is reviewed on Jan. 29.

Under European Union rules introduced after the global financial crisis, S&P publishes its rating reviews according to a schedule although it can make ad hoc changes in special circumstances. It did so last February, when it cut ratings for oil-exporting Bahrain, Kazakhstan, Oman, Venezuela and Congo all at once and changed outlooks on Saudi Arabia and Nigeria to negative before downgrading them later in the year.

Kraemer said the latest dive in oil prices might mean that kind of mass downgrade is seen again.

"The last leg down (in oil) has been pretty significant," he said. "So what we have to do is, if you remember last year we brought some reviews forward, quite a number actually, we have to decide whether we do that again.

"We come to that decision by looking at the new reality."