NEW YORK - The venture units of oil firms Saudi Aramco and Italy's ENI have joined the world's largest passenger carrier United Airlines to invest in British start-up OXCCU's efforts to slash the prohibitively high cost of lower-carbon aviation fuel, the companies said on Wednesday.

Aviation produces around 2% of the world's planet-warming emissions and the sector's own targets to emit no more than can be absorbed by natural sinks like forests or other technologies by 2050 pose a particularly daunting challenge.

The $22.7 million investment, led by U.S.-based investor Clean Energy Ventures, will go to Oxford University-affiliated scientists at OXCCU, one of several companies that have been searching for ways to replace kerosene and gasoline in plane engines.

OXCCU says it can make fuel by combining carbon dioxide captured from industry or power plants with hydrogen made using renewably sourced electricity. Its breakthrough is in using an iron-based catalyst to do this in one step, replacing the pricier two-stage process that is usually needed to bring about the chemical reaction.

Streamlining the process in this way knocks 50% off the capital cost and produces fewer byproducts, the companies said in a statement.

"This cutting-edge solution could be a cost-effective pathway for United to reach our commitment of net-zero carbon emissions by 2050, without relying on traditional carbon offsets," said United Airlines Ventures President Michael Leskinen.

(Reporting by Isla Binnie in New York Editing by Matthew Lewis)