Hong Kong has overtaken Luanda, Angola, to become the costliest city in the world for expats, according to an annual survey released by Mercer on Wednesday.

After topping Mercer's Cost of Living Report for three consecutive years, Luanda was narrowly edged by Hong Kong this year, owing to a stronger HK dollar.

The survey by Mercer, a consulting group, compares the cost of more than 200 items in more than 200 cities such as housing, food, transportation and entertainment.

Mercer takes New York as the base for comparison and measures currency movements against the US dollar, and Hong Kong's dollar, which is pegged to the greenback, has appreciated significantly over the past year.

"Generally speaking, prices remained stable across the world," Bruno Rocquemont of Mercer France said, attributing steep rises or falls in cities' rankings chiefly to exchange rate fluctuations.

A strong yen propelled Tokyo six places higher to become the world's fifth-most expensive expat destination, behind Zurich and Singapore in third and fourth place.

The cost of living in several US cities also rose on the back of the strong dollar.

Shanghai ranked seventh and Beijing was 10th.

Conversely, cities in countries whose currencies have slumped became more affordable, with Moscow recording one of the most spectacular price slides, falling from the 17th costliest city for expats to 67th.

At the bottom of the list of 209 cities was Namibia's capital, Windhoek.