FUKUSHIMA, Japan >> Japan prepared another risky venting of radioactive gas to relieve a new spike in pressure in one of its troubled nuclear reactors Sunday, a setback in efforts to bring the crippled, leaking plant under control just after some signs of improvement.

The planned release into the air of what officials said would be a densely radioactive cloud comes as traces of radiation are turning up well beyond the leaking Fukushima Dai-ichi plant after its cooling systems were knocked by the massive March 11 quake and tsunami on Japan’s northeast coast.

Radiation has seeped into the food supply, with spinach and milk from as far as 75 miles (120 kilometers) showing levels of iodine in excess of safety limits. Minuscule amounts are being found in tap water in Tokyo and rainfall and dust over a wider area. Taiwan even reported receiving a batch of contaminated fava beans imported from Japan.

“I’m worried, really worried,” said Mayumi Mizutani, a 58-year-old Tokyo resident shopping for bottled water at a neighborhood supermarket out of concern for her visiting 2-year-old grandchild. “We’re afraid because it’s possible our grandchild could get cancer,” she said.

The rising pressure at one of the reactors in the tsunami-damaged nuclear complex dealt a setback to the government just as it claimed progress in a spiraling crisis that has compounded recovery from the catastrophic natural disasters. On Saturday, officials cited headway in reconnecting power supplies at two of the plant’s six reactors and in cooling other reactors and fuel storage pools by pouring water on them.

After appearing to have stabilized, the plant’s Unit 3 reactor became troublesome again Sunday.

Pressure rose inside the vessel that contains the reactor core, necessitating a tricky venting of the gases inside to relieve the buildup, officials with the nuclear safety and the plant’s operator said.

The venting is an “unavoidable measure to protect the containment vessel,” nuclear safety agency official Hidehiko Nishiyama told reporters. He warned that a larger amount of radiation would have to be released than when similar venting was done a week ago because more nuclear fuel has degraded since then.

Nishiyama said experts were hoping to filter the released gas to reduce radiation, otherwise safety agency officials said “dry venting” could release 100 times more iodine as well as the radioactive elements krypton and xenon.

Growing concerns about radiation add to the overwhelming chain of disasters Japan has struggled with since the 9.0-magnitude quake. The quake spawned a tsunami that ravaged the northeastern coast, killing more than 8,100 people, leaving 12,000 people missing, and displacing another 452,000, who are living in shelters.

Fuel, food and water remain scarce for a 10th day in the disaster. The government in recent days have acknowledged being caught ill-prepared by an enormous disaster that the prime minister has called the worst crisis since World War II and that required an immediate, full-scale response.

In the latest admission, another nuclear safety official said Sunday that the government only belatedly realized the need to give potassium iodide to those living within 12 miles (20 kilometers) of the nuclear complex.

The pills help reduce the chances of thyroid cancer, one of the diseases that may develop from radiation exposure. The official, Kazuma Yokota, said the explosion that occurred while venting the plant’s Unit 3 reactor last Sunday should have triggered the distribution. But the order only came three days later.

“We should have made this decision and announced it sooner,” Yokota told reporters at the emergency command center in the city of Fukushima. “It is true that we had not foreseen a disaster of these proportions. We had not practiced or trained for something this bad. We must admit that we were not fully prepared.”

Contamination of food and water compounds the government’s difficulties, heightening the broader public’s sense of dread about safety. Consumers in markets snapped up bottled water, shunned spinach from Ibaraki — the prefecture where the tainted spinach was found — and overall express concern about food safety.

Experts have said the amounts of iodine detected in milk, spinach and water pose no discernible risks to public health unless consumed in enormous quantities over a long period of time.

Drinking one liter of water with the iodine at Thursday’s levels is the equivalent of receiving one-eighty-eighth of the radiation from a chest X-ray, said Kazuma Yokota, a spokesman for the prefecture’s disaster response headquarters.

No contamination has been reported in Japan’s main food export — seafood — worth about $3.3 billion a year, less than 0.5 percent of its total exports. But the island of Taiwan, just to the south and a huge market for Japanese goods, said Sunday that radiation had been detected in a batch of Japanese peas but at levels too small to harm human health.