China scrambled jets on Friday in response to two U.S. spy planes and 10 Japanese aircraft, including F-15 fighters, entering its new air defense zone over the East China Sea, state news agency Xinhua said, raising the stakes in a standoff with the United States, Japan and South Korea.
The jets were scrambled for effective monitoring, Xinhua cited air force spokesman Shen Jinke as saying. The report gave no further details.
Japan and South Korea flew military aircraft through the zone, which includes the skies over islands at the heart of a territorial dispute between Japan and China, the two countries said on Thursday, while Washington sent two unarmed B-52 bombers into the airspace earlier this week in a sign of support for its ally Japan. None of those aircraft informed China.
The Pentagon has declined to offer specifics on any additional U.S. flights and it neither confirmed nor denied the Chinese report of two U.S. spy aircraft entering the zone.
One U.S. defense official, speaking to Reuters on condition of anonymity, said only the U.S. military was still flying routine missions in the region, including reconnaissance and surveillance flights.
Xinhua earlier said China had sent several fighter jets and an early warning aircraft into the new air defense zone.
China last week announced that foreign aircraft passing through it - including passenger planes - would have to identify themselves to Chinese authorities.
The Chinese patrol mission, conducted on Thursday, was "a defensive measure and in line with international common practices", Xinhua reported Shen as saying.
The aircraft, including Russian-designed Su-30 fighter jets, conducted routine patrols and monitored targets in the zone, Shen said.
"China's air force is on high alert and will take measures to deal with diverse air threats to firmly protect the security of the country's airspace," he said.
However, Defense Ministry spokesman Yang Yujun said it was "incorrect" to suggest China would shoot down aircraft which entered the zone without first identifying themselves. He did not elaborate.
Japan's Chief Cabinet Secretary Yoshihide Suga said on Friday he did not know if Chinese planes were in the zone but added there was no change to Japan's sense of alertness.
Ties between China and Japan have been strained for months by the dispute over the islands in the East China Sea, called the Diaoyu by China and the Senkaku by Japan. Washington takes no position on the sovereignty of the islands but recognizes Tokyo's administrative control and says the U.S.-Japan security pact applies to them.
Europe's top diplomat, Catherine Ashton, said the European Union was concerned about China's decision to establish the new air defense zone as well as its announcement of "emergency defense measures" if other parties did not comply.
"This development heightens the risk of escalation and contributes to raising tensions in the region," Ashton said. "The EU calls on all sides to exercise caution and restraint."
China's Foreign Ministry spokesman Qin Gang criticized Ashton's remarks, saying China hoped the EU could treat the situation "objectively and rationally".
"Actually, Madam Ashton should know that some European countries also have air defense identification zones," Qin said. "I don't know if this leads to tensions in the European regional situation. European countries can have air defense identification zones. Why can't China?"
Asked to clarify China's expectations for what information airlines were expected to report, Qin said: "International law does not have clear rules on what kind of flight or airplane should apply", adding that each country made its own rules.
"Therefore, China's method does not violate international law and accords with international practice," he said.
China's Foreign Ministry said on Thursday that since the zone had come into force there had been no impact on the safe operation of international civilian flights, although it added that China "hoped" airlines would cooperate.
Japan's two biggest airlines have defied the identification order since Wednesday at the request of the Japanese government.
Although there are risks of a confrontation in the zone, U.S. and Chinese military officials have stepped up communication with each other in recent years and are in regular contact to avoid accidental clashes.
U.S. Vice President Joe Biden is visiting China, Japan and South Korea next week, and will try to ease tensions over the issue, senior U.S. officials said.
"We decline to comment on Chinese flights, but the United States will continue to partner with our allies and operate in the area as normal," a Pentagon spokesman said.
China's Defense Ministry has said that it was aware of the U.S., Japanese and South Korean military aircraft in the zone and had tracked them all.
Ties between China and Japan, often tense, have increasingly been frayed in recent years by regional rivalry, mutual mistrust over military intentions and what China feels is Japan's lack of contrition over its brutal occupation of parts of China before and during World War Two.
In a show of support for the military, Chinese President Xi Jinping visited a base in Jinan in eastern China, where he said "military training is critical to beef up the PLA's (People's Liberation Army) war capacities", according to the Xinhau news agency.
Xi did not make direct mention of the East China Sea air defense zone.
"Though life is becoming better, history can't be forgotten and those who made sacrifices for (the) new China's founding must be remembered," Xinhua quoted Xi as saying in a separate report.
The Global Times, an influential tabloid published by the ruling Communist Party's official People's Daily newspaper, praised the government for its calm response in the face of "provocations", saying China would not target the United States in the zone as long as it "does not go too far".
But it warned Japan it could expect a robust response if it continued to fly military aircraft in the zone.
"If the trend continues, there will likely be frictions and confrontations and even a collision in the air ... It is therefore an urgent task for China to further train its air force to make full preparation for potential conflicts," it wrote in an editorial on Friday.