The United Nations (UN) agency, the International Civil Aviation Organization (ICAO), approved a landmark agreement on 6 October to slow the growth of emissions from commercial flights, Reuters reports.

The global carbon offsetting system would be voluntary between 2021 to 2026 and mandatory from 2027, and applies to states with larger aviation industries. Under the system, airlines would have to buy carbon credits from particular environmental projects to offset any growth in emissions. It would apply to international passenger and cargo flights and business jets generating more than 10,000 tonnes of emissions/year.

A small number of countries have objected to the agreement, but the president of the ICAO’s governing council said they would not derail the plan.

Sixty-five countries accounting for more than 80% of aviation activities had agreed to participate in the first voluntary phase, which surpassed the ICAO’s expectations, Reuters said.

Russia and India said they would not participate in the voluntary phase, arguing it was unfair for emerging countries. China agreed to take part and Brazil voiced support although it did not commit to joining in 2021.

Reuters said that according to ICAO figures, the deal would cost airlines between US$1.5bn and US$6.2bn in 2025, and no more than 1.8% of industry revenues by 2035.

The International Air Transport Association vice president Paul Steele acknowledged that this was a cost for the industry but said “we believe it’s a manageable cost”.

Talks were continuing on the technical details, the report said, including the types of offset credits the agreement would accept.

Some environmentalists said the agreement would not meet its own goals and would only require airlines to offset 75% of growth after 2021, or one quarter of total international traffic.