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Which countries are the most and least corrupt?
Karam Al-Masri | AFP | Getty Images

The bloody civil war in Syria is having yet another impact: The nation is now considered one of the most corrupt in the world, according to newly released data from the anti-corruption group Transparency International.

The organization's annual "Corruption Perceptions Index," first published in 1995, is one of the most closely watched barometers of the issue. The group surveyed experts on public sector corruption in 177 countries, grading each nation on a scale of zero to 100, with zero being "highly corrupt" and 100 being "very clean."

Syria has never been considered particularly virtuous, but growing attention to the business dealings of President Bashar al-Assad and his associates helped its score plunge to 17 from 26 in 2012. The country now ranks near the bottom-tied with Turkmenistan and Uzbekistan for 168th place. It ranked 144th last year.

(Read more: Corruption Worsens Amid Deep Distrust of Government )

But no nation has a monopoly on corruption, according to the study, which found that 70 percent of the countries scored 50 or below.

"The 'Corruption Perceptions Index 2013' demonstrates that all countries still face the threat of corruption at all levels of government, from the issuing of local permits to the enforcement of laws and regulations," Huguette Labelle, chair of Transparency International, said in a statement.

America's ranking

The U.S. turned in a mediocre performance, according to the study. The world's largest economy scored 73 in this year's index-identical to last year-which puts it in a tie with Uruguay for 19th place. Canada, Germany, Great Britain and Japan are among countries considered cleaner than the United States.

(Read more: Nobel Prize winner warns of US stock market bubble )

Transparency International and others have criticized the U.S. for relatively lax controls on money laundering. Other issues considered include campaign finance and government contracting. The U.S. has never finished higher than 14th (in 2000) and has come in as low as 24th (2011), though the organization says year-to-year comparisons can be misleading because of changes in methodology.

This year's least-corrupt countries are Denmark and New Zealand, which both scored 91. The most corrupt are Afghanistan, North Korea and Somalia, each scoring a mere 8 out of 100 points.

Russia matched last year's score of 28 points, finishing in a nine-way tie for 127th place. China improved by one point, for a score of 40, tying with Greece for 80th place.

That score is a marked improvement from a year ago, when Greece was in the depths of a financial crisis. The resulting reforms may have helped boost last year's score of 36 points, but it is still perceived to be the most corrupt country in the euro zone, the study said.