Tiny Singapore now has the world's most powerful passport, according to a new ranking, with its citizens able to travel to the greatest number of countries visa-free.

Passport Index, which keeps track of how useable such documents are, said the city-state grabbed the top spot after Paraguay removed restrictions for Singaporeans.

That means the approximately 3.4 million holders of Singaporean passports can now travel to 159 countries either without a visa at all, or can have one issued on arrival.

Germany came in second place, with its citizens able to visit 158 countries without a visa, while Sweden and South Korea tied for third.

The US passport was in sixth place, alongside Malaysia, Ireland and Canada.

Afghanistan came bottom of the list with visa-free access to just 22 countries.

Passport Index said the US passport's usefulness has fallen since President Donald Trump took office, with Turkey and the Central African Republic becoming the most recent countries to revoke their visa-free entry for holders.

Passport Index ranks passports worldwide based on the cross-border access a holder has. It was developed by Canada-based global consultancy Arton Capital.

"For the first time ever, an Asian country has the most powerful passport in the world," Philippe May, managing director of Arton Capital's Singapore office, said in a statement.

"It is a testament of Singapore's inclusive diplomatic relations and effective foreign policy."