Security agents in Gabon are investigating several top civil servants, including an advisor to the president, for illegal trafficking in a precious hardwood, a source close to the government said Friday.

Some of the officials have been taken into custody by the General Directorate for Research (DGR) agency in the past week, while others were questioned and later released, the source told AFP.

A special court set up to deal with the abuse of protected wildlife resources in the densely forested tropical nation has opened a "preliminary inquiry" into illegal exports of Kevazingo, a rare hardwood much prized in Asia, added the source.

DGR agents this week raided the ministry of water and forests in Gabon's capital Libreville, taking about 10 officials into custody, the source said, asking not to be named.

Those under investigation include former minister of water and forests Nelson Messone, who has been an advisor to President Ali Bongo Ondimba since a government reshuffle in September. Messone was questioned in his office at the presidential headquarters.

Minister for the Protection of the Environment Flore Mistoul has meanwhile decided to "suspend temporarily and as a preventive measure the exploitation of Kevanzingo across the whole territory" while making an inventory of the precious trees with a view to better protecting them.

Environmental associations have reported that about 20 people, including several Chinese nationals and two provincial forest managers, were arrested near Makokou in northeast Gabon in connection with the illegal exploitation of Kevazingo wood.

"The ramifications of these investigations and tip-offs led to the arrest of people at the ministry," said the source close to the government, who added that the northern provinces of Woleu Ntem and Ogooue-Ivindo are the "epicentre of a huge forestry crime racket".

Also known as Bubinga and locally regarded as sacred, Kevazingo trees take many years to mature and can grow to more than 40 metres (130 feet) tall. The Japanese and Chinese use the timber to make furniture and items such as specialty guitars, which they sell to the West.

Gabon legally exports almost 18,000 cubic metres (635,000 cubic feet) of Kevazingo timber each year, under strict supervision. Rising demand has inflated the price. In China, one cubic metre can sell from between 1,500 to 3,000 euros (almost $1,600-$3,200), according to non-governmental organisation Conservation Justice.

For months the NGO has denounced "a veritable mafia organisation" behind illegal trafficking in Kevazinga, along with money laundering.