Fuels made out of biomass aren't always emitting fewer greenhouse gases than regular fossil fuels, top European scientists say in a draft study that is likely to add fire to an ongoing debate about the actual effectiveness of the so-called biofuels in cutting CO2 emissions from transport.

"It is widely assumed that bioenergy is carbon neutral. However, this assumption is flawed as it ignores the fact that using land to produce plants for energy typically means that this land is not available to produce plants for other purposes, including carbon sequestration," reads the draft, written by the scientific committee of the European Environment Agency and seen by Dow Jones Newswires Wednesday.

The study, a tool that policymakers at the European Commission can use to make proposals to regulate the sector, adds to the debate on the actual effectiveness of biofuels in reducing CO2 emissions.

The European Union has decided, as part of its effort to build a greener economy and fight global warming, that 10% of energy used in transport should come from renewable sources, which mainly means biofuels. However, since that target was set almost three years ago, skepticism has grown about whether these fuels are really cutting CO2 emissions as expected.