The isotope, cesium 137, was measured in one village by the International Atomic Energy Agency at a level exceeding the standard that the Soviet Union used as a gauge to recommend abandoning land surrounding the Chernobyl reactor, and at another location not precisely identified by the agency at more than double the Soviet standard.

The measurements, reported Wednesday, would not be high enough to cause acute radiation illness, but far exceed standards for the general public designed to cut the risks of cancer.

While the amount measured would not pose an immediate danger, the annual dose would be too high to allow people to keep living there, according to Edwin Lyman, a senior scientist at the Union of Concerned Scientists, an American organization that is often critical of nuclear safety rules. Cesium persists in the environment for centuries, losing half its strength every 30 years.

The International Atomic Energy Agency stressed that levels of contamination varied considerably from place to place. Experts said the measurement might represent a “hot spot” and might not be representative of larger areas, though that remains to be seen.

Dr. Lyman said that if a plume of contaminants had drifted with the wind, a large amount could have been dumped in one spot by a rainstorm. “I think it’s not surprising that there would be local concentrations that high,” he said. But Japan should expand the evacuation zone, now set at 19 miles, he said, and the International Atomic Energy Agency should release data faster. The measurements were made between
March 18 and March 26, the agency said.

Yukio Edano, the chief cabinet secretary, said on Thursday morning that officials were studying the contamination levels, but that there were currently no plans to expand the evacuation zone.

Land can be cleaned up by scraping off the contaminated dirt, or paving it over. Asked if land abandonment was likely, Dr. Lyman said, “It depends on the cost of cleanup and how much people are willing to spend.”
Japan, experts noted, is far more densely settled than the Chernobyl region of Ukraine, where a reactor explosion in 1986 contaminated large areas.

The international team, using a measure of radioactivity called the becquerel, found as much as 3.7 million becquerels per square meter; the standard used at Chernobyl was 1.48 million.

In another development, seawater near the plant showed significantly higher levels of radioactive iodine than in recent days, Japan’s nuclear safety agency reported Wednesday. In addition, the operator of the plant acknowledged for the first time that at least four of the six reactors at the multibillion-dollar complex would have to be scrapped.

Hidehiko Nishiyama, deputy director general of the Nuclear and Industrial Safety Agency, said Wednesday that seawater collected about 300 yards from the Fukushima Daiichi station was found to contain iodine 131 at 3,355 times the safety standard. On Sunday, a test a mile north showed 1,150 times the maximum level, and a test the day before showed 1,250 times the limit in seawater taken from a monitoring station at the plant.

The level of radioactive iodine 131 in the waters off the Daiichi plant continued to increase on Thursday, rising to 4,385 times the statutory limit, Mr. Nishiyama said at a news conference. The increases raise the possibility that contaminants from the plant are continuously leaking into the sea, he said.

Workers have been dousing the nuclear fuel rods with seawater and now from freshwater sources to prevent full meltdowns, but they have had to release harmful amounts of radioactive steam into the atmosphere, and their efforts have set off leaks of highly contaminated water.

Iodine 131, one of the radioactive byproducts of nuclear fission, can accumulate in the thyroid and cause cancer. But unlike cesium 137, it degrades relatively rapidly, becoming half as potent every eight days. The risk can be diminished by banning fishing.

Mr. Nishiyama said the new readings posed no immediate threat to public health, and no fishing was occurring in the area.

Regarding the international agency’s findings, he said on Thursday, “One sample is over the limit, so we have to continue monitoring the situation.” As to whether the evacuation zone should be expanded, he said, “We’ll be considering that.”

The plant’s operator, the Tokyo Electric Power Company, acknowledged that it would write off Reactors 1 through 4.

Matthew L. Wald reported from Washington, and David Jolly from Tokyo. Moshe Komata and Ayasa Aizawa contributed reporting from Tokyo, and Kevin Drew from Hong Kong.