The number of domestic air passengers has declined over 20 percent since South Korea opened its first high-speed train service earlier this year.

In a report to the National Assembly on Sunday, the state-run Korea Airports Corporation said 2.8 million people took flights from Seoul to Busan, Daegu and Gwangju during the April-July period, 24 percent less flights compared to the same period last year.

The number of flights on the three routes also stood at 21,500 for the same period, a decline of 20 percent from 2003.

The number of air passengers traveling from Seoul to the southern island of Jeju took an upturn within the same period, posting 3.8 million, up 1.7 percent from last year.

The Korea Train Express, or KTX, began operations of its high-speed trains between major cities in April.