The number of passengers that U.S. airlines carried so far this year is on the rise, according to a new report.

The Bureau of Transportation Statistics (BTS), the research branch of the U.S. Department of Transportation, said Thursday that domestic and international passenger seats filled in 2013 have noticeably outpaced passenger loads reached in 2012, even as airlines logged a lower number of flights and while relying more on smaller regional jets.

The report shows that 600,000 more passengers traveled in March 2013 than traveled in 2012 even though there were 2.3 percent fewer flights or 18,800 fewer in the same year-over-year comparison. During the the first quarter of 2013, 900,000 more passengers flew than did in 2012 despite a 3.4 percent reduction in flights or 76,600 fewer.

Passenger trip length also rose by 0.9 percent in both the year-over-year and the quarter-over-quarter comparisons.

Delta Air Lines, Inc. (NYSE:DAL) led in seats filled across both time measures while Atlanta Hartsfield-Jackson International Airport, which is also Delta's hub, led in passenger traffic.

The study calculates what BTS calls load factor, which measures revenue per mile as a proportion of seat availability per mile.