GENEVA - The International Air Transport Association (IATA) announced Thursday that January saw the slowest year-over-year air passenger increase in nearly four years.

IATA said in a statement that the results were affected by temporary factors including the later timing of the Lunar New Year in 2018 as well as less favorable comparisons with the strong upward traffic trend seen in late 2016-early 2017.

"Despite the slower start, economic momentum is supporting rising passenger demand in 2018," said Alexandre de Juniac, IATA's director general and CEO. "That said, concerns over a possible trade war involving the US could have a serious dampening effect on global market confidence, spilling over into demand for air travel," the IATA CEO noted.

Traffic rose 4.6 percent in the first month of the year compared to January 2017.

IATA estimates the impact of the later Lunar New Year-related travel period holiday represented around two-fifths of the slowdown in year-over-year growth for the month.

International passenger demand growth slowed to 4.4 percent in January, from 6.1 percent in December, with all regions recording growth, led by Latin America and Europe.

Asia-Pacific carriers recorded a demand increase of 4.6 percent compared to January 2017, which was a 46-month low, said IATA.

This largely was owing to the impact of the later Lunar New Year, which fell in mid-February this year.

European carriers' international traffic bucked the trend slightly, climbing 6.0 percent in January compared to the period a year earlier, up from 5.8 percent growth in December 2017.

The region was the only one to see an acceleration in traffic compared to the prior month. This is being supported by the buoyant economic conditions in the region, said IATA.

North American airlines experienced a 3.5 percent rise in traffic over a year ago.

The relatively healthy economic backdrop in the region is helping support outbound demand but IATA said this is being partly offset by a negative impact on inbound traffic to the United States.