SEOUL (Reuters) - The International Air Transport Association said on Sunday it expects the amount of sustainable aviation fuel produced to double in 2025 to reach 2 million tonnes, representing 0.7% of airlines' fuel consumption.

Influential industry body IATA has increasingly been warning that airlines will struggle to meet their sustainability goals, and has described the production of SAF - which is more expensive than conventional jet fuel - as disappointingly slow.

IATA Director General Willie Walsh said that while the production increase was encouraging, the relatively small amount will add $4.4 billion globally to aviation's fuel bill.

"The pace of progress in ramping up production and gaining efficiencies to reduce costs must accelerate," Walsh said in a statement.

The broader aviation sector agreed in 2021 to target net-zero emissions in 2050 based mainly on a gradual switch to SAF, which is made from waste oil and biomass.

Airlines are at odds with energy companies over scarce supplies of SAF, while also pointing the finger at plane makers Airbus and Boeing ( BA ) over delays in delivering more fuel-efficient jets.