Tokyo residents carrying radiation-detection equipment have found small hot spots in several areas of the city, prompting Japanese officials to promise more detailed government monitoring of radiation levels in the country’s most populous region.

Tokyo is more than 125 miles from the Fukushima Daiichi nuclear facility that was heavily damaged by the March 11 earthquake and tsunami, but residents reported Thursday that they had found two small areas with radiation levels higher than some within the 12-mile evacuation zone. One of the two elevated readings, local officials said at a briefing, might be linked to radioactive materials that were stored in a nearby home.

Separately, a soil sample in Yokohama, just south of Tokyo, was found this week to contain strontium-90, an isotope that can accumulate in bones and cause cancer. It was the first time that strontium, with a half-life of 29 years, was detected more than 60 miles from the plant. The strontium was found atop an apartment building in a measurement made by a private agency at a resident’s request.

Further testing, the officials said, is needed to clarify where the radioactive isotopes came from and whether radiation has traveled farther from the plant than initially expected.

Seven months after a triple meltdown at the coastal nuclear plant, Japan is still trying to get a sense of the extent of the radioactive fallout. Tracking contamination is complicated. Radiation can be carried by the wind, but its spread can also be blocked by mountains or suppressed by rain.

Sometimes, levels vary drastically within a few feet. Residents who doubt the accuracy of the government’s monitoring are performing their own tests. Even if the little hot spots pose no health risk, the search for them reflects a nationwide concern.

The two latest readings came in Setagaya, a ward in western Tokyo, and in Funabashi, a city in Chiba prefecture, east of Tokyo. Japanese media reported that the hot spot in Setagaya, measured at 3.35 microsieverts per hour, was probably unrelated to the nuclear disaster, because radium-226 was found in the sample.

“When we measured the area surrounding the hot spot, the reading did not register very high,” Setagaya Mayor Nobuto Hosaka said. “The reading was exceptionally high around the wall of an old house.”

But in Funabashi, city workers cordoned off an area near a children’s theme park after a citizens group detected a level of 5.82 microsieverts per hour, according to the Kyodo news service. Nuclear experts say mini-hot spots present minimal health risk. Somebody exposed to that radiation level for a year, though, would receive about 51 millisieverts, more than twice the Japanese government’s annual limit of 20 millisieverts.

Apart from the latest radiation detections, Japan faces a massive decontamination job that will require years of work and billions of dollars. Officials say an area of 925 square miles must be decontaminated by a combination of scrubbing and topsoil removal.

The towns closest to the plant, located along the coast in Fukushima prefecture, will not be habitable for decades. Last week, health-care workers began conducting checkups on more than 300,000 children in Fukushima.

Special correspondent Ayako Mie in Tokyo contributed to this report.