U.S. backs market scheme for aviation emissions from 2020 MONTREAL (Reuters) - The United States supports a proposed market-based system to curb carbon emissions from the global aviation sector starting in 2020, but warned that concerns over measures to lower emissions in the interim threaten to undercut the broadly supported goal, a U.S. official said. Negotiators from over 190 countries gathered at the United Nations' International Civil Aviation Organization's triennial assembly in Montreal went over details from a proposal to curb greenhouse emissions from the aviation industry as agreed by ICAO's 36-member governing council earlier this month. Many countries have expressed support for a global market-based measure to reduce airline emissions, the details of which would be decided in 2016 for a proposed 2020 launch. As a result, aviation could become the first major industry sector to launch a global market system to curb carbon emissions. Country representatives said on Thursday they are wary of a section of the plan that addresses how interim national and regional measures that charge foreign air carriers for their emissions would operate until 2020. The United States' delegation supports countries and regions that want to have interim regulations in place before a global mechanism is created, but only if they limit measures to their respective airspace, the U.S. official said. It is concerned, however, that the proposed resolution would exempt a large number of countries from complying with those interim measures. Under the current wording, around 160 of 191 ICAO states would be exempt. Throughout this process, the U.S. government and the U.S. airline industry have strongly supported ICAO efforts to develop a global market-based measure to reduce greenhouse gas emissions. That position has not changed," a U.S. official told Reuters ahead of Thursday's discussions. While we support the concept of de minimis exemptions, the current approach raises serious concerns for the United States. Julie Oettinger, the lead U.S. negotiator and assistant administrator of the Federal Aviation Authority, said in remarks to the assembly on Thursday that exemptions should be granted to airline operators rather than states. "A single operator may have very high relative levels of activity, while the same may not be true for its state of registry," she said, adding that such an approach would avoid market distortions, an ICAO requirement. Poorer countries, however, have required an exemption from any interim measures in exchange for supporting the resolution, said Englebert Zoa Etundi, Cameroon's representative to ICAO, who represents the African bloc of negotiators. The measure would protect African air passengers and its burgeoning airline industry and the bloc supports the proposal as it is currently worded, he said. Other representatives of developing countries agreed that the resolution should protect the "special circumstances" of poorer states in remarks before the assembly on Thursday. India's negotiator said at the assembly that the resolution should not allow countries or regions to impose their own market systems before a global market mechanism is developed. Deleting the provision would avoid a "Pandora's box of considerations" and avoid a barrage of potential retaliatory measures, he added. The European Union, currently the only region that has a market-based system to charge fees on emissions that would apply to foreign air carriers, said any changes to the current proposal could unravel, "a very delicate balance that has the support of the majority of ICAO." Arijandas Sliupas, Lithuania's deputy transport minister, said on behalf of the European Union's 44 member states that, although the agreement is "not perfect, if forms the single package we have all been looking for." From the aviation industry's perspective, allowing exemptions, even on interim measures, could set a worrying precedent. "If such a large set of countries can get a blanket exception on market-based measures, we are concerned that this could be a slippery slope to requests for exemption on other taxes and charges, and even safety and/or security measures," said Nancy Young, vice president for environmental affairs for Airlines for America, the U.S. airlines' lobby group. Some industry representatives are also concerned that, if so many countries are exempt now, they will seek to protect their exemptions if a global scheme starts up in 2020. Environmental groups said there is wide support for a global deal after years of tough negotiations and warned that concerns over just a few provisions should not stop wider progress. Annie Petsonk, international counsel for the Environmental Defense Fund, said the goal is to launch the global emissions cap process and deliver it by 2016. We are at a crucial moment in the history of ICAO's efforts to tackle the climate issue. The credibility of this institution and of the governments who lead it - including the U.S. and the EU - is on the line," Petsonk added. Reporting by Valerie Volcovici.; Editing by Ros Krasny, G Crosse and Andre Grenon