"We want to make sure that the risks are properly understood," Stas Burgiel, policy director of GISP, told Reuters. The report was issued to coincide with a May 19-30 UN conference on protecting biodiversity in Bonn.

Invasive species can overtake new habitats, causing billions of dollars of damage, if they lack competitors or pests that keep them in check at home.

The study by GISP, which groups scientists around the world, adds to worries about side-effects of biofuels including that they push up food prices or add pressure on farmers to clear forests and other land to produce energy.

Many countries favour biofuels as alternatives to oil, costing more than $125 a barrel, and to curb climate change blamed on greenhouse gases emitted by burning fossil fuels.

The report said countries should be wary of the West Asian giant reed arundo donax, for instance, which is being introduced as a biofuel to the United States. "Naturally flammable, it increases the likelihood of wildfires -- a threat to both humans and native species in places such as California," it said.

The American mesquite tree, under consideration for biofuel production, is known as the "Devil's Tree" in Ethiopia because it has taken over larger than expected areas since it was introduced in the 1970s as a drought-resistant species.

Its 10 cm (3 inch) thorns injure both people and livestock.

And the African oil palm, recommended for biodiesel, "has already become invasive in parts of Brazil, turning areas of threatened forest from a rich mix of trees and plant life into a homogenous layer of palm leaves," it said.

The report listed nine crops of low risk as biofuels because they were not known to be aggressive invaders -- including sunflowers, soya, sugar cane, cotton and wheat.

"You don't often see wheat growing outside a wheat field. It has become so domesticated that its has to be planted," said Jeffrey Howard of the International Union for Conservation of Nature which backs GISP.

"But the giant reed is going to cause a problem almost anywhere you take it," he told Reuters.

Plants used as biofuels are part of a wider problem of invasive species ranging from rats to jellyfish.

A GISP statement said experts estimate damage from invasive species at up to five percent of the global economy. "The US alone spends $120 billion annually on the control and impacts of more than 800 invasive species infestations," It said.

The report recommended that governments should assess risks before introducing new species. Australia and New Zealand, or instance, vet new crops under tight rules on biosecurity.

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/