Negotiations for a freeze, or even a reduction, in greenhouse gas emissions from civil aviation are making progress, Europe's transport commissioner said Friday, adding she is feeling "cautiously optimistic."

The International Civil Aviation Organization (ICAO), a UN agency, opened its triennial assembly in Montreal on Tuesday and is meeting there until October 7, with a climate-centered agenda.

"After four days of discussions," said EU transport commissioner Violeta Bulc.

"I am pleased with the positive momentum emerging from ICAO's Assembly, and cautiously optimistic that there will be a positive outcome," the Slovenian EU commissioner said.

Carbon dioxide emissions from the aviation sector are not part of the Paris accord (COP21) aimed at limiting the global temperature rise to +2 degrees Celsius -- or if possible +1.5 degrees -- by 2050 compared to the pre-industrial era.

The 191 member states are seeking agreement on a proposed global mechanism of compensation for international aviation emissions. It goes by the name of CORSIA, or the Carbon Offset and Reduction Scheme for International Aviation.

The objective is to ensure that the volume of aviation emissions in 2035 is no higher, and hopefully lower, than the 2019 and 2020 levels.

To date, 63 nations representing 84 percent of aviation traffic -- including the biggest polluters such as the European Union, the United States and China -- have agreed to take part in this system starting in 2021, according to the ICAO.

"There are still some open issues that need to be solved for all states to be onboard," Bulc said. She said the 28-member EU would "continue to defend the current text, in order to prevent further weakening" of the proposed measure.

A source close to the discussions told AFP that informal talks were under way to address the final differences, stressing that Latin American and African countries were expressing "considerable" support for the proposal.

"Without an effective market-based measure to reduce aviation emissions, it will be impossible to keep temperatures within 1.5 degrees as we all agreed to do in Paris," said Mike Halferty, transport minister of the Marshall Islands, whose Pacific Ocean territory is threatened by rising sea levels.

"This is why I have personally made the long journey here to help fight for this measure."

He added: "We simply cannot afford to wait another three years for the aviation sector to take action."