Elsewhere, the paper said that the United States was not the target of the new zone.

Responding to the situation, the State Department said, “We have urged the Chinese to exercise caution and restraint, and we are consulting with Japan and other affected parties throughout the region.”

Analysts have said that China’s declaration of the new zone is meant to whittle away at Japan’s hold on the islands. But the unexpected move is also seen as another attempt by an increasingly assertive China to establish itself as the dominant regional power, displacing the United States.

China had seemed to be stepping back this week from its original harsh tone, when it said aircraft entering the airspace needed to file flight plans in advance or face the possibility of military action. On Wednesday, a Foreign Ministry spokesman said China would decide on a case-by-case basis how strongly to respond to those who break its rules.

In a further clarification of its original stance, the People’s Liberation Army said Thursday that the new air zone was “not a territorial airspace” and did not mean that China would take immediate military action against aircraft that entered the zone.

At a monthly briefing for Chinese reporters, a spokesman, Yang Yujun, said it was “incorrect” to suggest China would shoot down planes in the zone. On Thursday, Japan’s top government spokesman, Yoshihide Suga, said that the Chinese had not been notified of the Japanese flights, and reported that China had not scrambled its fighter jets to intercept them.

The South Korean government announced that it, too, had flown aircraft through the zone without alerting Beijing. Chinese officials said they had monitored the flight by what the South Koreans described as a surveillance aircraft. Like Japan, South Korea claims sovereignty over some territory in seas beneath the airspace, but Seoul enjoys warmer ties with Beijing than does Tokyo.

During a previously scheduled defense meeting on Thursday, South Korea asked China to change the boundaries of the new zone, according to the South Korean Yonhap news agency. But China rejected the request, said a spokesman for South Korea’s Defense Ministry, Kim Min-seok, according to the Yonhap report.