(CNN) -- Nearly half the world's primate species are in danger of extinction, according to a report released Wednesday by a major conservation group.

The main threat facing primates -- including apes, monkeys, and lemurs -- is tropical forest destruction, with the illegal wildlife trade and commercial bush meat hunting also playing roles.

Scientists say primates are humankind's closest animal relatives.

Of the world's 634 primate species, 48 percent are threatened with extinction, according to the report, issued by the International Union for Conservation of Nature. The Switzerland-based group calls itself the world's oldest global environmental organization.

"This report makes for very alarming reading," said Christoph Schwitzer, an adviser to the group, in a statement. "Support and action to help save these species is vital if we are to avoid losing these wonderful animals forever."

A handful of primate species count populations in the dozens. For example, there are just 60 to 70 Asian monkeys known as golden headed langurs, found only on an island in Vietnam's Gulf of Tonkin.

There are fewer than 100 remaining northern sportive lemurs, which live in Madagascar, and around 110 eastern black crested gibbons, found in northeastern Vietnam.

The report, which details the 25 most endangered primates, has been issued biannually by the International Union for Conservation of Nature since 2000. It will be formally released Thursday at Britain's Bristol Zoo Gardens.