The rate at which the world is losing its forests has halved, but an area of woodland the size of South Africa has still been lost since 1990, a UN report revealed Monday.

Improvement has been seen around the globe, even in the key tropical rainforests of South America and Africa, according to a surprisingly upbeat Forest Resources Assessment (FRA), which is released every five years

Despite the good news, it points out that since 1990, the world had lost forests covering some 129 million hectares -- an area the size of South Africa.

"Even though, globally, the extent of the world's forest continues to decline... the rate of net forest loss has been cut by over 50 percent," said the report by the UN's Food and Agriculture Organization (FAO).

The assessment was released at the World Forestry Congress in the South African port city of Durban, host to the 14th edition of the conference.

"FRA 2015 shows a very encouraging tendency towards a reduction in the rates of deforestation and carbon emissions from forests and increases in capacity for sustainable forest management," said FAO director-general Jose Graziano da Silva.

"The direction of change is positive, with many impressive examples of progress in all regions of the world."

WWF International director for forests, Rod Taylor, said the report presented "good news at one level, but the question is how sustainable that is".

"Even with the reduced rate we still have unacceptable levels of forest loss," Taylor told AFP.

WWF said that without "bold and urgent action" up to 170 million hectares -- the size of Germany, France, Spain and Portugal combined -- could be wiped out in the next 20 years.

Greenpeace International executive director Kumi Naidoo told the congress that "little progress has been made in fulfilling pledges to... completely eliminate deforestation."

- Planted forests increase -

Apart from offering oxygen, fuel and building material, trees store important quantities of carbon, which, if released, contribute to global warming.

Halting deforestation is a key focus of UN negotiations for a global pact to limit disastrous climate change caused by greenhouse gas emissions.

The UN talks are designed to secure a deal to be signed by world leaders in Paris in December.

In 1990 the world had 4,128 million hectares of forest covering 31.6 percent of the global land area, the forest report said.

By 2015 this had decreased to 3,999 million hectares, covering 30.6 percent -- a net loss of some 129 million hectares.

The net annual rate of loss -- which takes into account the planting of new forests -- has slowed from -0.18 percent in the 1990s to -0.08 percent over the last five years.

Planted forest area has increased by more than 110 million hectares since 1990 and now accounts for seven percent of the world's forest area.

The biggest loss of forests occurred in the tropics, particularly in South America and Africa, although even there the rate of loss "has decreased substantially in the past five years", the report said.

Natural forest will probably continue to decline, but "due to growing demand for forest products and environmental services, the area of planted forests is likely to continue to increase in coming years".

The conclusions raised questions of whether alarm bells sounded over forest loss have been overplayed, but the report's team leader, Kenneth MacDicken, said the FRA had led a change in attitude over deforestation.

"The FRA has since 1948 reported forest area change -- including the loss of forest area in the tropics.

"Actions in response to this information have helped slow the rate of forest loss -- and in some countries have resulted in increased forest area," MacDicken told AFP from his base in Rome.

Better information from new forest inventories had also "greatly improved our understanding of forest change", he said.