The comments came after Hanoi said Chinese vessels rammed its patrol ships and turned water cannon on them near a controversial drilling rig in a disputed patch of the South China Sea.
At the same time, Japan and China continue to face off in their own territorial row over a small island grouping in the East China Sea and amid claims that Beijing is becoming increasingly assertive.
"We have strong concerns as there is information that many Vietnamese vessels were damaged and some people were injured," Japan's Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo.
"We recognise this incident is part of China's unilateral and provocative maritime activities," he said.
Suga said China should explain to Vietnam and the international community the basis on which it was acting and added Japan strongly wants China to refrain from provocative moves and "act in a self-restrained manner".
Hanoi said yesterday that Chinese ships protecting a deep-water drilling rig in disputed waters had used water cannon to attack Vietnamese patrol vessels and had repeatedly rammed them, injuring six people.
Vietnam deployed patrol vessels after the China Maritime Safety Administration issued a navigational warning on its website saying it would be drilling close to the Paracel Islands -- which are controlled by China but claimed by Vietnam.
The two countries, who fought a brief border war in 1979, have been locked in a longstanding territorial dispute over the waters, and frequently trade diplomatic barbs over oil exploration, fishing rights and the ownership of the Spratly and Paracel Islands.