Somalia, South Sudan and Syria have been ranked the world's most corrupt nations in an international report.

Denmark, New Zealand and Finland topped the list of 'clean' countries in the Corruption Perceptions Index released by Watchdog group Transparency International.

The table measures perceived public-sector corruption in 180 nations and uses a scale on which 100 is seen as very clean and zero is very corrupt.

The report urged governments to address problems with political party financing as it emerged the US and several other leading superpowers are struggling to keep up momentum in the fight against corruption.

America's score of 69 was two points lower than a year earlier and its worst score for eight years, Transparency International said. The US was ranked 23rd, a one-place drop from last year.

The report cited challenges including 'threats to its system of checks and balances' and 'the ever-increasing influence of special interests in government.' It also noted the launching of impeachment proceedings against U.S. President Donald Trump.

The top ranking countries are New Zealand and Denmark, with scores of 87 each, followed by Finland (86), Singapore (85), Sweden (85) and Switzerland (85)

The average regional scores, with top and bottom performers in each region

The index is calculated using 13 different data sources that provide perceptions of public sector corruption from business people and country experts.

According to the report, released on Thursday, the most corrupt nation went to Somalia, with just nine points.

South Sudan, Syria and Yemen finished just above it, behind a group of countries sharing 173rd place that included Afghanistan and Venezuela.

Denmark, last year's top performer, was joined in a tie for first place this time by New Zealand. Both had 87 points. They were followed closely by Finland.

Singapore, Sweden and Switzerland tied for fourth. Norway, the Netherlands, and Germany and Luxembourg - tied for ninth - completed the top 10.

A Somali woman observes the aftermath of a car bomb explosion in Mogadishu in December 2019 (File image)

New Zealand Prime Minister Jacinda Ardern pictured in December 2019. Denmark, last year's top performer, was joined in a tie for first place this time by New Zealand

Canada lost four points to 77, placing 12th. It tied with Britain, which lost three points - as did France, which tied with the U.S.

Of the other countries in the Group of Seven leading industrial powers, Germany and Japan, which ranked 20th, were static, while only Italy gained one point to 53, putting it in 51st place.

More than two-thirds of countries around the world scored below 50 and the average score was only 43, Transparency International said. It said countries that perform well on the index have stronger enforcement of campaign-finance regulations.

The United States' score of 69 was two points lower than a year earlier and its worst score for eight years

'Frustration with government corruption and lack of trust in institutions speaks to a need for greater political integrity,' said the group's head, Delia Ferreira Rubio. 'Governments must urgently address the corrupting role of big money in political party financing and the undue influence it exerts on our political systems.'

The report also noted that 'integrity at home does not always translate into integrity abroad, and multiple scandals in 2019 demonstrated that transnational corruption is often facilitated, enabled and perpetuated by seemingly clean Nordic countries.'

Notable gains were made this year by Armenia and Angola, which each gained seven points - to 42 (77th place) and 26 (146th) respectively.