The victims were tallied at 6,570 as of this year, down 19 percent from 8,075 recorded in 2016, according to the data by the Ministry of Interior and Safety.
It marks a drastic fall from the previous figures of 11,717 in 2014 and 9,937 in 2015. The figure has more than halved from 13,854 in 2013.
The compiled data are based on the government grants paid to the surviving victims. It pays an annual 800,000 won ($696) to every victim as part of medical support.
Their declining numbers attest to the fact that the victims -- mostly in their 80s and 90s -- are dying without getting fair compensation in time, civic groups said.
During World War II, hundreds of thousands of Koreans were mobilized as forced laborers by the Japanese military to toil under dangerous and poor working conditions without pay.
The surviving Korean victims and their advocates have urged Tokyo and the companies responsible for the abuses to admit to and compensate for their wrongful acts. Japan claims the damages have been covered by a 1965 treaty with Seoul on resolving colonial-era issues.
A total of 15 lawsuits in Korea have been filed since the early 2000s against Japan's Mitsubishi Heavy Industries, one of the firms involved in forced labor. But the litigations have dragged on for nearly two decades mainly because of the defendant's dilatory attitude.
"Many of them, who are elderly with physical limitations, are reluctant to join the litigation because they are discouraged about whether they will ever get the court ruling," said Lee Kook-un, head of a local activist group. "The Supreme Court must make up its mind soon."
A 2000 damages suit against Mitsubishi has been delayed until Wednesday, while all five plaintiffs are now deceased. Two out of the four plaintiffs that were engaged in a suit against Nippon Steel & Sumimoto Metal Corp. have also passed away.
Currently, three cases are pending at the top court. (Yonhap)