SEOUL -- Chief of South Korea's ruling Democratic Party said Friday that a new deal with Japan is needed over wartime sex slavery victims to make the deal acceptable to the victims.

Choo Mi-ae, chairperson of the Democratic Party, told a party meeting that the 2015 deal was not an agreement as it was aimed to cover up truth, referring to the 2015 deal between then South Korean government under ousted President Park Geun-hye and the Japanese government led by Prime Minister Shinzo Abe.

The deal was reached on Dec 28, 2015 to finally and irreversibly settle the issue over South Korea's comfort women victims in return for Japan's offer of 1 billion yen ($9 million) to a foundation supporting the victims.

Comfort women euphemistically refer to the Korean girls and young women who were kidnapped, forced or duped into sex servitude for Japanese military brothels during World War II. Up to 200,000 women, mostly from the Korean Peninsula, served as sex slaves during the devastating war, according to historians.

A task force of Seoul's foreign ministry announced its review result over the 2015 deal earlier this week, discovering a secret agreement between the two countries. It angered the victims and advocacy groups supporting the victims.

President Moon Jae-in said the wartime sex slavery issue cannot be resolved by the 2015 deal.

The governing party chief said historical issues should be dealt with in a way not to damage truth and principle.

Some of South Korean netizens said on the Internet that the government should return the 1 billion-yen fund back to Japan to start renegotiations on the wartime crime against humanity.

Japan claimed that the 2015 deal should be implemented, indicating no intention to renegotiate the deal.