Jeju Air Co., a South Korean low-cost carrier, said Tuesday it will start flights to Vietnam's Nha Trang starting next month.

The route is the latest move by the budget carrier to expand into Southeast Asia and Japan not only to absorb the growing travel demand but also to offset sharp declines in passengers from China.

Beijing banned the sale of travel packages to South Korea starting March 15 in retaliation against Seoul's deployment of the Terminal High Altitude Area Defense system. 

It has argued the X-band radar system that comes with THAAD could spy on its military though Seoul and Washington have said the system is purely aimed at countering North Korea's missile threats.

Jeju Air said it will start operating the Incheon-Nha Trang route four times a week from July 26.

In 2016, the country's six budget carriers -- Jin Air, Jeju Air, Air Busan Co., Air Seoul Inc., Eastar Jet and T'way Air Co. -- transported 56.8 percent of the passengers on domestic routes and 19.6 percent of passengers traveling abroad. The figures mark a gain from 54.6 percent and 14.6 percent, respectively, from a year earlier, according to the transport ministry.

Last year, passengers on domestic routes jumped 11 percent on-year to 30.91 million and passengers on international routes climbed 19 percent to 73 million, government data showed. (Yonhap)