Fiscal reforms are just part of the picture for Middle East economies working to adapt to an era of lower crude oil prices, Moody's Investors Service said.

Moody's said last week the drop in crude oil prices, off more than 50 percent since the middle of 2014, has undermined the economy and finances for members of the Gulf Cooperation Council: Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Bahrain and Oman.

According to Moody's, the nominal gross domestic product in Saudi Arabia alone declined around 13 percent last year and should shrink another 5 percent in 2016. In a new report published from Dubai, the ratings agency said that, even though the regional economies are introducing a variety of fiscal measures meant to reflect lower oil revenues, the reforms will only partially compensate for the decline in crude oil prices since 2014.

"As such, Moody's expects that fiscal and external constraints will persist beyond 2016," the ratings agency said.

A January forecast from the World Bank said some oil exporters in the region could see GDP growth of 6.2 percent this year. Saudi Arabia's growth, meanwhile, stays below 3 percent at least through 2018.

The report notes that governments like Bahrain, Oman and Saudi Arabia, which have faced human rights pressure, may be dealing with complex underlying social issues when trying to implement economic policy reforms.

Outside of the GCC, an IMF report found the Iranian economy is outperforming the region as a whole mainly because of higher oil production and the lifting of international economic sanctions.

For the GCC, Moody's analyst Mathias Angonin, who wrote the report on the region, said regional economies will continue to be strained by the weak energy market.

"Low oil prices are testing even strong institutions," he said in a statement.