BEIJING — China said it had sent warplanes to patrol its newly declared maritime air defense identification zone on Thursday, while Japan and South Korea have also sent surveillance aircraft into the area.

Air force spokesman Shen Jinke said several fighter jets and an early warning aircraft had been sent on “defensive” air patrols in the zone, to “strengthen the monitoring of aerial targets,” the Xinhua state news agency reported.

The United States has joined with many of China’s neighbors in condemning China’s decision to establish the zone , and also defied Beijing by sending two B-52 bombers through the zone on Tuesday.

Experts say China’s decision to establish the zone -- bolstered by a threat that any noncommercial aircraft entering the zone without informing it could face “defensive emergency measures” – has ratcheted up an already tense stand-off with Japan over a disputed island chain, and raised the possibility of military conflict.

But Beijing had initially reacted calmly to the U.S. challenge on Wednesday, simply noting that it had identified and monitored the American warplanes. That muted response drew criticism from Chinese netizens, and even from state media.

The nationalist Global Times newspaper said the United States had engaged China in a “war of public opinion” but that Beijing had “failed to make a timely and ideal response.”

“Beijing needs to reform its information release mechanism to win the psychological battles waged by Washington and Tokyo,” it said in an editorial .

Colonel Shen said the air force would remain on high alert and would take “relevant measures according to different air threats” to defend the country’s airspace, Xinhua reported.

In another editorial , the Global Times said Japan, not the United States, was the target of the new zone, and suggested the enforcement of the zone would be selective.

“If the US does not go too far, we will not target it in safeguarding our air defense zone,” it wrote. “What we should do at present is to firmly counter provocative actions from Japan.”

The latest flights intensify the game of dare being played above Asia’s contested maritime territory . Experts said China had established the zone to bolster its claims to a chain of tiny, rocky islands administered by Japan, and to match a similar air defense identification zone long established by its rival.

But they said the decision could have backfired , uniting several of China’s neighbors in condemnation and providing the United States a perfect opportunity to demonstrate its commitment to ensuring stability in the Asia-Pacific region.

A Japanese government spokesman said Thursday that its Lockheed Martin-made turboprop patrol planes have been conducting routine flights in China’s air defense identification zone since Beijing’s declaration. The spokesman did not say specifically when the flights have taken place or how many there have been. Japan has not been notifying China of its activities.

Japan’s Defense Ministry did not confirm the flights, but one official, requesting anonymity to describe the situation, said that Japan is “conducting the same monitoring activity as before, and we will not change or restrict such activities.”

South Korea’s flight took place Wednesday near the area of a South Korean maritime research center, built atop a submerged rock that both Seoul and Beijing contest.

The flight marks a “clear sign that Seoul will not recognize the new airspace claim,” South Korea’s Yonhap news agency said in its report. Yonhap also reported Thursday that China had rejected Seoul’s request to redraw its air defense identification zone and eliminate an overlap with the zone of South Korea. The South in turn said it may expand its own zone.

In Asia’s waters, territorial disputes go back decades or centuries and draw in nearly every nation in the region. The nastiest dispute of late has been between Japan and China over several uninhabited islets and rocks known as the Senkaku in Japanese and the Diaoyu in Chinese.

Japan infuriated China last year by purchasing several of those islets from a private landowner, and China has since increased its surveillance — both with vessels and aircraft — around those islands. Japan has frequently scrambled its own fighter jets in response.

U.S. officials said China’s unilaterally announced air defense identification zone needlessly raised tensions between Asia’s two largest economies. In his trip next week through the region, Vice President Biden plans to convey those concerns to China, a senior Obama administration official told reporters on Wednesday.

“There is an emerging pattern of behavior by China that is unsettling to China’s own neighbors,” the official said. Biden will raise questions “about how China operates in international space and how China deals with areas of disagreement with its neighbors.”

Harlan reported from Seoul. Li Qi contributed