Japan and South Korea say they have flown aircraft through China's newly declared air defense zone over the South China Sea, following the example of U.S. aircraft earlier this week.
Tokyo and Seoul said Thursday that they did not notify China before crossing into the disputed air space.
China declared an Air Defense Identification Zone (ADIZ) over disputed waters in the East China Sea on Saturday. It said all foreign civilian and military aircraft flying in the zone must identify themselves and follow Beijing's instructions or face unspecified "emergency measures."
The Chinese zone includes air space above a resource-rich chain of uninhabited islands claimed by China and controlled by Japan, a regional rival.
But the United States said it flew two unarmed B-52 bombers through the zone on Monday without notifying China. Japanese airlines operating in the area stopped submitting flight plans to Beijing on Wednesday, at the request of the Japanese government.
China's defense ministry said Wednesday it monitored the U.S. military planes while they transited the zone. It also insisted that China has the capability to exercise "effective control" of the air space.
When asked whether China will take tougher measures if there are further such incidents, Chinese Foreign Ministry spokesman Qin Gang said Beijing will "respond accordingly depending on the different circumstances and the threat levels that we may face."
The Chinese government's muted statements on the issue prompted many Internet users to describe the Chinese aerial zone as a farce. Those sympathetic to the government said stronger action is needed in future.
In a Weibo message monitored by Reuters, outspoken retired Chinese Major General Luo Yuan called for the zone to be "enforced to the full" and said "no country must think that they can ... leave things to chance."
Chinese officials have said Beijing established the ADIZ in order to exercise its "right" to defend national sovereignty. They also have said China is acting like other nations that have created aerial notification zones in international airspace.
The United States and Japan have their own aerial zones, but only require foreign aircraft to identify themselves if those planes intend to pass through U.S. and Japanese national airspace.