Where does the U.S. rank when it comes to retirement security?

America lags behind more than a dozen and a half countries when it comes to ensuring Americans a financially comfortable retirement.

The U.S. ranks No. 19 in a new global ranking for retirement security by Natixis Global Asset Management, which is based in Paris and Boston. The money management firm noted that while the U.S. has held the 19th spot on the list for its third consecutive year, the country's position is "fragile" given some negative trends.

Those include widening income inequality, a rapidly aging population, and a shift toward a retirement system that requires workers to become their own chief investment officers through self-directed 401(k)s and IRAs, rather than the traditional pension-based system. On top of that, the U.S. spends more on health care than any other country, yet medical care doesn't reach all citizens thanks to the large gap in earnings between the top and the poorest.

"We're seeing that individuals will have to shoulder more of the financial burden by saving and investing more effectively to ensure financial security in retirement," John Hailer, president and chief executive officer for Natixis Global Asset Management in the Americas and Asia, said in a statement.

European countries ranging from Switzerland -- which Natixis ranks No. 1 for retirement security -- and Norway (No. 2) are more likely to provide a comfortable old age for their citizens, thanks to broad access to health care, well-developed and growing economies, and low income inequality, the report noted.

Two Asian countries -- Japan (No. 17) and Korea (No. 14) -- ranked higher than the U.S., as well as some small European countries, including Iceland (No. 4) and Luxembourg (No. 11.)

Although the U.S. has one of the world's highest per capita incomes, the country ranks low for income equality compared with other developed countries, Natixis noted. "This could be a potential explanation for a comparatively low life expectancy despite the world's highest health expenditure per capita, and could be a cause for concern for retirees," it added.

The U.S. wealth gap is increasingly sparking concern among economists and policy makers, thanks to what ratings agency Standard & Poor's had warned is approaching an "extreme" threshold. With the top 1 percent of earners grabbing most of the income and wealth gains, that's leaving less for the remaining 99 percent, which is dampening economic growth, S&P said last year.

To be sure, widening inequality isn't only a problem in the U.S., given that the top 1 percent will own most of the world's wealth by 2016. According to charity Oxfam, the world's richest citizens have seen their control of global wealth rise from 44 percent in in 2009 to 48 percent in 2014.

With many Americans struggling with stagnant wages, it's no surprise that retirement savings remains low on their list of priorities. Four out of 10 Americans haven't started saving for retirement, a BlackRock study found in October. On top of that, many of those who are putting money away are making an investment error that could eat into their retirement cushion: keeping the majority of their savings in cash.

The Natixis study analyzed four categories to come up with its global rankings: health, material well-being, finances and quality of life. Added up, those trends can help predict whether a country's citizens can expect to be financially secure when they retire.

The countries in the bottom of the list include many African and Middle Eastern nations, with the West African country of Togo ranking last. Many Sub-Saharan countries "lack modern infrastructure and have non-existent or underdeveloped healthcare systems. They have some of the lowest levels of income per capita and are often burdened with substantial barriers to economic development, such as high levels of inflation and sovereign debt," the report noted.

Below are the top 20 countries ranked by retirement security:

1. Switzerland
2. Norway
3. Australia
4. Iceland
5. Netherlands
6. Sweden
7. Denmark
8. Austria
9. Germany
10. New Zealand
11. Luxembourg
12. Canada
13. Finland
14. Republic of Korea
15. Czech Republic
16. Belgium
17. Japan
18. France
19. U.S.
20. Slovenia