TOKYO—Japanese Prime Minister Shinzo Abe took a significant step toward removing six-decade-old constraints on Japan's military, a move that could enable Tokyo to play a greater security role in an increasingly tense East Asia.

Citing the recent standoff in disputed South China Sea waters between China and Vietnam, Mr. Abe said Japan needed more freedom of action in overseas conflicts, despite its pacifist constitution.

"In the South China Sea, even as we speak, confrontations between countries are continuing because of unilateral action backed by force," he said at a news conference Thursday. "It could very well be our problem."

Mr. Abe said his government would kick off discussions to determine whether to change the interpretation of the constitution to permit the use of "collective self-defense." The shift would allow Japanese troops, known as the Self-Defense Forces, to aid its allies even if Japan itself isn't attacked.

To illustrate the need for collective self-defense, Mr. Abe pointed to a panel depicting a U.S. ship carrying Japanese children being attacked by an unidentified continental country west of Japan labeled "the aggressor." In such a scenario, he said, the government couldn't send forces to help the children—unless it changed a policy that dates to the founding of Japan's modern military in the early 1950s.