Airline fuel efficiency on transatlantic flights has improved by one percent a year since 2014 as carriers modernized their fleets, but the industry still lags its own climate goals, according to a study released on Wednesday by the International Council on Clean Transportation.

But the study's findings were challenged later Wednesday by an aviation industry group which argued that carriers are actually ahead of their targets.

According to the ICCT study, the industry's average fuel efficiency, as measured by how much fuel is used to transport passengers, improved to 34 passenger kilometers per liter of fuel from 33 between 2014 and 2017 as carriers opted for modern aircraft with lower fuel burn and operated fuller planes.

Airlines have been switching to more fuel-efficient planes to mitigate the impact of high oil prices on their margins.

The aviation industry has also set a non-binding goal of capping emissions from international flights at 2020 levels, despite rising passenger traffic as demand for global travel climbs.

Haldane Dodd, spokesman for the Air Transport Action Group (ATAG), which counts airlines among its members, said the industry is achieving a 2.1 percent rolling average fleet fuel efficiency improvement for all flights.

That figure is ahead of a goal of 1.5 percent efficiency improvements per year, Dodd said by email.

"Rankings of airline efficiency based on assumptions and modelled estimates are not a useful tool for either the public or policymakers, as they present an inaccurate picture of the operation," Dodd said.

The ICCT compared the fuel efficiency of nonstop passenger flights between North America and Europe by 20 major airlines. While the study only examined transatlantic flights, the authors argued the results showed industry lagged its goals.

Flights to and from European states are expected to account for 36 percent of all carbon emissions from global aviation in 2020, the ICCT said citing data from the International Civil Aviation Organization (ICAO).

"New policies to accelerate investments in more fuel-efficient aircraft and operations are critical if industry is to meet its long-term climate goals," said Dan Rutherford, aviation program director for the ICCT, a U.S.-based independent non-profit research organization.

According to the study, budget carrier Norwegian Air Shuttle ASA was ranked first of 20 transatlantic carriers for fuel efficiency, while British Airways, part of the International Airlines Group, came in last.

British Airways on Wednesday said its fuel efficiency per passenger "appears lower," because on such routes the carrier has a "greater share of the premium market," which has fewer seats.