Pensions systems across the world are under pressure due to increasing lifespans and other factors, but some nations continue to provide quality retirement benefits for their works, according to a new report.

The 11th annual Melbourne Mercer Global Pensions Index, released this week, studied 37 nations, representing about 63% of the world's population, and graded them on the quality of their pension systems using 40 data indicators.

The United States, however, wasn't among the top finishers.

The U.S. pension system got an overall grade of C+, placing 16th among all nations, just behind the U.K. and Hong Kong, and one place ahead of Malaysia. The report said the U.S. system would be better if it raised the minimum pension for low-income workers, increased mandatory contributions and reduced worker assets to funds before retirement, among other suggestions.

Researchers noted, however, that all nations in the report have room for improvement, with factors such as the low-growth and low-interest environment putting pressure on pensions systems all over the world.

The report noted, however, that the aging of the population is one of the top issues pension systems are dealing with.

"Pension systems around the world, including social security systems and private sector arrangements, are now under more pressure than ever before. Significant ageing of the population in many countries is a fact of life," David Knox, a senior partner at Mercer, wrote in the report.

The following nations all received grades of B or more and were, in ascending order, deemed by Mercer to have the world's best pension systems:

13. Germany

Germany got an overall B ratings for its pension system, but received only a D for the "sustainability" subcategory. That means researchers had doubts about the nation's ability to financially sustain what is now a high-quality system. The report recommended that Germany increase its level of funded contributions to increase asset levels over time.