The eldest son of Lotte Group's founder stepped up his legal attack against his younger brother by suing one of his executives and four Japanese units, escalating a long-running struggle for control over the South Korean retail giant.

Shin Dong-joo, who was dismissed from his positions at Lotte's Japanese units in January, said he filed a lawsuit against Takayuki Tsukuda, president of unlisted Lotte Holdings Co., which is key to controlling Lotte Group, in the Tokyo District Court. He also filed a separate case against four group units — Lotte Co., Lotte Shoji Co., Lotte Real Estate Co. and Lotte Bussan Co. — and is seeking compensation for damages.

"I was removed from the executive positions by incredibly unfair means," Shin Dong-joo said at a media briefing in Tokyo on Thursday, a month after their 93-year-old father sued his younger son and other board members of Lotte Holdings in South Korea to nullify the patriarch's own July ouster from the company.

The brothers have been fighting for control of the department stores-to-hotels conglomerate which their father Shin Kyuk-ho started in Japan in 1948. The ongoing dispute comes at a time Shin Dong-bin, the younger of the brothers, is planning a public listing of the group's Hotel Lotte Co. unit, which he had said would be completed by the first half of 2016.

Lotte Group is seeking to raise 7 trillion won ($6 billion) by selling 35 percent of Hotel Lotte, with 15 percent in new shares and 20 percent in existing shares, MoneyToday reported last month, citing an unidentified senior official from the conglomerate.

Shin Dong-joo said he would team up with his father to resolve the issue "as soon as possible" and that he wants to bring Lotte Group's management to what "it's supposed to be," without further elaborating.