A local court has opened a formal damage suit that ten former South Korean sex slaves for Japan's World War II soldiers filed against Tokyo, after failing to find solutions through a mediation process.

The Seoul Central District Court on Wednesday ended the mediation process after the Japanese government repeatedly rejected to participate in the procedure, court officials said.

The decision came a few days after Seoul and Tokyo reached a breakthrough deal in which the Japanese government apologized and offered reparations of 1 billion yen ($8.3 million) to the victims. South Korea agreed to end the dispute once and for all if Japan fully implements the deal.

The victims, however, oppose the agreement saying that it lacks acknowledging legal responsibility.

"When a case is not suitable for mediation, (the court) decides not to mediate (and open a trial)," a court official said.

Mediation is a way of finding solutions without former court trials. When an agreement is not reached amicably, former trials can be reopened.

In 2013, a dozen elderly South Korean women filed for mediation to the court seeking 100 million won ($85,000) in compensation each.

Still, Japan returned the relevant documents to the court, insisting that the South Korean court does not have jurisdiction over the Japanese government.

The court set June 15 and July 13 as dates for mediation and asked the Japanese government to send its representative, but no Japanese official showed up.

Two of the 12 elderly women have since died, lowering the number of plaintiffs to 10.

After about two years of mediation efforts, the elderly women submitted documents to the court twice, seeking a formal suit against Japan.

Now a trial has begun, but there is still the high possibility that the Japanese government will not respond to it, citing the Monday deal.

Even without the participation of the Japanese government, the court can hand down a ruling.

Still, even if the court rules in favor of the plaintiffs, a separate case needs to be filed with a Japanese court to execute the compensation decision.

Only 46 former sex slaves, euphemistically called "comfort women," remain, as many others have died over time. Historians estimate more than 200,000 women, mostly Koreans, were forced into sexual slavery at front-line Japanese brothels during World War II. (Yonhap)