Japan says it intends to strengthen its aerial surveillance, hoping to prevent a repeat of an unprecedented incident Thursday when a Chinese government plane flew near disputed isles in the East China Sea.
The Japanese Air Self Defense Force scrambled eight F-15 fighter jets after a coast guard vessel radioed that a Chinese aircraft was flying just south of the largest disputed island.
Japanese land-based radar failed to detect the low flying turboprop surveillance plane from China.
China's foreign ministry spokesman Hong Lei responded to reporters' questions Friday about the incident.
Hong says the foreign ministry has demanded many times Japan cease its "illegal activities in territorial aerospace and waters of the Diaoyu Islands, including withdrawing their aircraft." He adds that Japan has not responded and having a Chinese maritime surveillance plane protect China's territorial sovereignty is "totally normal."
Japanese authorities term unprecedented such a violation of their airspace by a Chinese aircraft.
Japan's chief cabinet secretary, Osamu Fujimura, says further such incidents will be dealt with firmly. Fujimura says the defense ministry is considering patrols by E2C early warning aircraft and AWACS control planes. Japan, he says, wants to utilize every measure it can to protect its airspace.
The provocative flight came during a week when China also sent a flotilla of navy ships near the islands, known as Senkaku in Japanese and Diaoyu in Chinese. The presence of the naval vessels follow what have become almost routine sailings near the territory by Chinese maritime surveillance ships.
The long-festering territorial dispute flared this year after Japan's central government moved to purchase the islands from a private Japanese owner.
That action was taken to keep the islands out of the hands of then Tokyo governor Shintaro Ishihara. He has since quit the post and formed a new political party.
Ishihara's fringe Japan Restoration Party is hoping to capture enough seats in Sunday's parliamentary election to have influence in forming the next government.
The governing Democratic Party of Japan is widely forecast to lose its grip on power. The largest conservative party, the Liberal Democratic Party, hopes it will capture enough seats outright to place its leader, former prime minister Shinzo Abe, back in his old job.
Abe is known for his hawkish stance towards China.
Although polls show voters, of which 40 percent say they are undecided on their party of preference, are primarily concerned with economic issues, political analysts assert public concern about the increasing Chinese patrols around the islands could give a boost to Abe's and Ishihara's parties.