(Adds details on airports, load factor, freight)
MONTREAL, April 6 (Reuters) - Global airline passenger traffic grew 6.8 percent in 2015, with the Middle East region growing the fastest, preliminary data released on Wednesday by the United Nations' civil aviation agency showed.
U.S. carriers were ranked the top three in the world by revenue passenger kilometer, American Airlines Group taking the top position, the International Civil Aviation Organization said in a statement.
Hartsfield-Jackson Atlanta International Airport was the world's busiest airport by number of passengers, followed by Beijing Capital International Airport and Dubois International Airport, ICAO said.
The global load factor, which measures how much of an airline's passenger carrying capacity is used, hit a 10-year high of 80.2 percent in 2015 because of improved air carrier optimization. North America recorded the highest load factor globally, at about 83.5 percent.
Global freight traffic grew more slowly last year, despite strong gains in passenger volume, reflecting stagnating world trade development, ICAO said. Global freight traffic grew 2.2 percent last year, compared with 4.9 percent in 2014. (Reporting By Allison Lampert; Editing by Jonathan Oatis and Grant McCool)