Japan protested to China today, after a Chinese government plane entered what Japan considers its airspace over disputed islets in the East China Sea.
The incident prompted Japan's military to scramble eight F-15 fighter jets, the Defence Ministry said. Japanese officials later said the Chinese aircraft had left the area.
Sino-Japanese relations took a tumble in September, after Japan bought the tiny islands - called Senkaku in Japan and Diaoyu in China - from a private Japanese owner.
"Despite our repeated warnings, Chinese government ships have entered out territorial waters for three days in an row," Japanese chief cabinet secretary Osama Fujimura said.
"It is extremely regrettable that, on top of that, an intrusion into our airspace has been committed in this way," he said, adding that Japan had formally protested through diplomatic channels.
A Defence Ministry spokesman said, as far as he knew, it was the first time this year that a Chinese plane had intruded into airspace near the islands.
The incident comes just days before a Japanese election that is expected to return to power the conservative Liberal Democratic Party (LDP), with hawkish former prime minister Shinzo Abe at the helm.
Abe has vowed to take a stern stance in the dispute over the islands, which are near potentially huge maritime gas reserves, and has said that the ruling Democratic Party's mishandling of its diplomacy had emboldened China.
Abe has also promised to boost spending on defence including on the coastguard.
Smaller Asian countries such as the Philippines have also become increasingly worried about Beijing's growing military assertiveness and its claims to disputed islands in the South China Sea.