TOKYO—Japanese Prime Minister Shinzo Abe stepped up his campaign to revisit Japan's wartime history, responding to criticism from Asian neighbors with a new round of comments that have further aggravated tensions with South Korea and China.

With emotions running high over visits by members of his cabinet to a wartime memorial, Mr. Abe stoked passions further by appearing to question whether Japan's World War II aggression and occupation of countries around the region could be labeled an "invasion."

"The definition of what constitutes an 'invasion' has yet to be established in academia or in the international community," Mr. Abe said in parliament on Tuesday, after a fellow lawmaker asked whether he supported a 1995 apology issued by Japan's prime minister at the time for Japan's colonial behavior. "Things that happened between nations will look different depending on which side you view them from."

The situation has become so prickly that South Korea's Foreign Ministry summoned Japan's ambassador Thursday to lodge a formal complaint about that comment—three days after the South Korean foreign minister canceled a planned trip to Tokyo over the weekend war-memorial visits.

"At this occasion, we expressed strong regrets with regards to the distorted understanding of history and anachronistic words from Japan, its government and politicians," South Korean Foreign Ministry spokesman Cho Tai-young said Thursday. "We hope Japan's leaders will reflect upon its colonization and invasion with an honest and modest mind-set and correct its anachronistic understanding and rhetoric," he added.