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The Chinese Ministry of Defense's account of the two incidents stood starkly at odds with one given on Tuesday by Japan's Ministry of Defense, which said that on Jan. 30 a Chinese military vessel trained a radar used to help direct weapons on a Japanese naval destroyer near the islands in the East China Sea. Japan also said that a Chinese frigate directed the same kind of radar at one of its military helicopters on Jan. 19.

Because using such "fire-control" radar can precede an attack, the Japanese defense minister. Itsunori Onodera, said that a misstep "could have pushed things into a dangerous situation."

China's first substantial response to the allegations amounted to a wholesale denial – which only deepened the puzzle of what happened, and who made any of the alleged decisions to use the radar. Japan promptly rejected the statement.

When Chinese naval vessels encountered the helicopter and destroyer in the East China Sea, their radar had "maintained normal observational alertness, and there was no use of fire-control radar," said a statement issued on the Chinese defense ministry's Web site on Friday. The statement was first issued by state media late on Thursday Beijing time. It did not explain what was meant by "normal observational alertness."

"The Japanese claim that Chinese naval vessel fire-control radars had aimed at a Japanese vessel and craft is out of step with the facts," said the Chinese defense ministry.

The Chinese defense ministry accompanied its denial with accusations that Japan was to blame for any unnervingly close encounters between their ships and aircraft in the East China Sea area.

"The Japanese side has recently incessantly issued falsehoods that distort and malign the normal combat preparedness training of Chinese military forces," it said. Japan was "deliberately creating a tense atmosphere and misleading international opinion," the Chinese defense ministry said.

The Chinese Foreign Ministry said on Thursday that a "competent department" was investigating the Japanese allegations.

The contested islands are called the Diaoyu in China and the Senkaku in Japan. They are controlled by Japan, but both China and Taiwan maintain that history and international law give them rightful claim.

Long-standing tensions over the disagreement flared in September, when the Japanese government purchased three of the five islands from a private owner, a step that China said amounted to a provocative denial of its territorial claims. Torrid and sometimes violent protests broke out in dozens of Chinese cities.

In the months since, the Chinese government has underscored its claim to the islands by sending government vessels and military ships and aircraft in their vicinity in a cat-and-mouse contest with Japanese Coast Guard ships. Tensions mounted in January, when both countries sent fighter jets over the East China Sea at the same time.

In Tokyo, Japan's Chief Cabinet Secretary Yoshihide Suga responded Friday at a news conference, saying, "We cannot accept China's explanation."

"We urge China to take sincere measures to prevent dangerous actions which could cause a contingency situation," he said.

For all China's vehemence, the statement by its defense ministry about the radar suggested that senior officials in Beijing want to avoid an escalating quarrel, said Denny Roy, a senior fellow at the East-West Center in Honolulu who researches security issues in the Asian region.

"I think it's a positive development that the Chinese would deny doing this, as opposed to saying, 'Yes we did it, and we'll do it again, and maybe we'll do more than that next time'," said Mr. Roy. "For the Chinese to not want to be portrayed as an aggressor, I think, is a good sign."

China's opaque and deeply secretive politics made it difficult to say whether any decision to use the fire-control radar came from the top of the Communist Party leadership or lower rungs of the military, Mr. Roy said. Many experts believe that "such a decision is not likely to be made by the local commander," he said.

"But that doesn't discount the possibility that somebody caught up in a situation could make the decision themselves," Mr. Roy said.

Bree Feng and Patrick Zuo provided research from Beijing.