The video-on-demand service is a fast-growing market for the pay TV industry, but opportunities are not going evenly to all operators, a report said Thursday.

The report released by Korea Information Society Development Institute showed VOD sales growing yearly since 2014, expanding from 573.39 billion won ($507 million) that year to 650.78 billion won in 2015 and to 709.38 billion won in 2016.

But not all operators gained from the growing business, according to the report. Internet protocol television took the major share, with its VOD sales growing 11.9 percent in 2015 and 9 percent in 2016, while sales for cable TV dropped 25.5 percent and 19.7 percent, respectively, in the cited years. VOD sales also fell for satellite TV operators by 0.1 percent and 7 percent.

Industry officials said cable and satellite TV networks are disadvantaged because the number of their subscribers who have access to VOD service is limited. In case of cable TV, only 54.6 percent of the users were subscribed to a package that includes VOD as of 2015. Subscribers of satellite TV need a separate receiver that can connect with the Internet in order to access VOD service.

All IPTV subscribers have two-way infrastructure that gives them VOD service without extra equipment.

The government is helping analog subscribers of cable TV switch to digital service, which in the process is expected to increase the availability of VOD. But officials in the cable TV business say government efforts are more focused on improving image resolution.

"A true digital service needs support for high-definition screens, multiple channels and two-way infrastructure," one official said. "To increase VOD sales, we need separate measures." (Yonhap)