Nearly 100 environmental and poverty fighting groups jointly released a letter Tuesday slamming a UN proposal that backs large-scale use of biofuels in commercial planes.
Extensive burning of biofuels would vastly expand the production of palm oil, which critics say drives deforestation, higher CO2 emissions and conflicts with indigenous peoples displaced from their land.
The UN's International Civil Aviation Organization (ICAO) "vision" plan calls for 128 million tonnes of biofuels to be used in jet engines every year by 2040, going up to 285 million tonnes -- half of all aviation fuel -- by 2050.
By comparison, some 82 million tonnes of biofuels a year are currently used in transport of all kinds worldwide.
Rapid scaling up of palm plantations could also eventually lead to price hikes for staple foods due to competition for arable land, recent research has shown.
"We therefore call on ICAO's Member States to oppose the promotion of biofuels for aviation," the NGOs said in the letter.
The ICAO proposal will be discussed at its three-day Conference on Aviation and Alternative Fuels in Mexico City, starting on October 11.
Fuel made from crops such as corn and palm oil were introduced decades ago as a "greener" alternative to gas and diesel.
Palm oil is the cheapest type of vegetable oil available in large quantities. It is mostly produced in Malaysia and Indonesia, but more recently in central Africa as well.
Greenhouse gas emissions from commercial aviation rose by nearly 90 percent between 1990 and 2014.
If the sector were a country, its CO2 output would be on a par with Germany or Indonesia, and place it among the top 15 carbon polluters worldwide.
Last October, the ICAO agreed that any growth in greenhouse gas emissions from commercial aircraft after 2020 would be offset by carbon-reduction schemes, such as planting forests that soak up CO2. "The biofuels hype should not be used as another greenwashing measure," said Carlos Calvo Ambel, a climate analyst at industry watchdog Transport & Environment, based in Brussels.
"Instead, ICAO should make aviation pay its fair share of taxes and promote measures that do reduce aviation emissions."
The international arms of ActionAid, Oxfam and Friends of the Earth were among the 96 signatories.