TOKYO—Tokyo's former envoy to China says the Japanese government erred in choosing to buy islands claimed by both Japan and China last fall, infuriating Beijing, and now both sides have no choice but to allow the issue to cool.

Uichiro Niwa told reporters on Monday that the purchase of the islands by Japan's central government was poorly timed and seemed driven by factors he could not explain.

Tokyo replaced Niwa, a former trading house executive, after ties between Beijing and Tokyo deteriorated amid the dispute over the uninhabited islands in the East China Sea.

Apart from confrontations between Chinese and Japanese vessels near the islands, both sides have scrambled fighter jets to trail each other's planes, raising the risk of missteps that could lead to a clash.