By Tom Watkins, CNN

Tokyo (CNN) -- A meltdown may be under way at one of Fukushima Daiichi's nuclear power reactors in northern Japan, an official with Japan's Nuclear and Industrial Safety Agency told CNN Sunday.

"There is a possibility, we see the possibility of a meltdown," said Toshihiro Bannai, director of the agency's international affairs office, in a telephone interview from the agency's headquarters in Tokyo. "At this point, we have still not confirmed that there is an actual meltdown, but there is a possibility."

A meltdown is a catastrophic failure of the reactor core, with a potential for widespread radiation release.

Though Bannai said engineers have been unable to get close enough to the core to know what's going on, he based his conclusion on the fact that they measured radioactive isotopes in the air Saturday night.

  Smoke rises from Fukushima Daiichi No 1 plant after a blast at the power station following Japan's earthquake and tsunami. Photograph: Staff/Reuters

"What we have seen is only the slight indication from a monitoring post of cesium and iodine," he said. Since then, he said, plant officials have injected sea water and boron into the plant in an effort to cool its nuclear fuel and stop any reactions.

"We have some confidence, to some extent, to make the situation to be stable status," he said. "We actually have very good confidence that we will resolve this."

A state of emergency has been declared for it and two other reactors at the same complex, which holds a total of six reactors, he said. Three are in a safe, shut-down state, he said. "The other two still have some cooling systems, but not enough capacity."

But Ichiro Fujisaki, Japan's ambassador to the United States, told CNN's Wolf Blitzer in Washington that he did not know of any evidence of a meltdown.

"We are working on it," he said. "We are getting information every hour on this issue."

And Japan's chief cabinet secretary, Yukio Edano, raised another possible issue. "Some of the readings in the measurement equipment were not accurate," he said without elaboration.

He noted that since the inside of the pressure vessel has been filled with sea water, radiation levels in the area have not risen, implying that the problem was not worsening.

The problems reported at two other reactors in the complex stemmed from a similar cause: an insufficient amount of water being pumped into the cores in an attempt to keep them cool.

To ease the pressure inside reactor No. 3, air containing "some minimal radioactive material" was being vented from its containment vessel, Edano said, expressing confidence in the outcome. "We believe that we can stabilize the situation of the reactor," he said.

In addition, nine residents near the site of an explosion at one of the plants were determined to have gotten radioactive material on their clothing and were being decontaminated, he said. "Now, we're trying to make sure they have not been internally exposed to radioactive material."

He described their exposure as "not hazardous to health."

Though the vast majority of people surrounding reactor No. 1 have been evacuated, 114 people remained, he said, but efforts were under way to get them out quickly.

Everyone within 3 kilometers of the No. 2 reactor has been evacuated; another 30,000 people within 10 kilometers of the plant were to be evacuated later Sunday morning, he said.

From CNN's Tom Watkins