MONTREAL — UN aviation experts are again discussing toughening emissions standards for commercial aircraft, less than six years before a previously agreed clampdown takes effect.

Support for a new emissions standard could put pressure on planemakers, which need years to adapt to rule changes due to long production cycles, to cease producing their least efficient models, two sources familiar with the talks said.

Experts from the United States and some European countries backed tougher emissions standards during a virtual gathering of the International Civil Aviation Organization (ICAO) group this week, according to working papers and sources.

One of the sources said ICAO’s Committee on Aviation Environmental Protection (CAEP) agreed on Thursday to draft new standards for civil aircraft, as part of broader efforts through 2025 to update rules for aircraft noise and emissions.

But it remains unclear when the proposed standards for commercial aircraft, such as those made by planemakers Boeing Co. and Airbus SE, would be drafted and take effect, and how stringent they would be, the source said.

“It’s a real struggle to see it all getting done by 2025,” the source said.

The meeting comes as ICAO is seeking broad agreement this fall on a long-term climate goal amid differences between Europe and China and growing pressure for aviation to curb emissions.

While any standard would take years to draft, win support from countries and wind its way through ICAO, the prospect of tougher emissions rules could potentially become one more headache for pandemic-weary planemakers.

“Any new standard creates pressure for planemakers,” said the second source. “What we don’t know is how much pressure.”

Montreal-based ICAO sets standards on everything from runway markings to crash investigations, which its 193 member states typically translate into regulatory requirements.

ICAO declined comment ahead of an official announcement.

ICAO’s governing council has already backed emissions rules that would be phased in for existing aircraft built from 2023, with a cut-off date of 2028 for planes that do not comply with the standard, unless exempted.

Boeing Co. has already said it is weighing an exemption for its 767-300F, a popular freighter model that would otherwise have to cease production in 2028. ICAO experts also supported the drafting of new standards for supersonic jets, the first source said. Aircraft makers wanted new noise and engine emissions standards for supersonic jets, to help the fledgling industry. — Allison Lampert/Reuters