TOKYO (AP) ― Highly toxic plutonium is seeping from the damaged nuclear power plant in Japan’s tsunami disaster zone into the soil outside, officials said Tuesday, further complicating the delicate operation to stabilize the overheated facility.

Plutonium has been detected in small amounts at several spots outside the Fukushima Daiichi power plant for the first time, plant operator Tokyo Electric Power Co. said.

Safety officials said the amounts were not a risk to humans but support suspicions that dangerously radioactive water is leaking from damaged nuclear fuel rods ― a worrying development in the race to bring the power plant under control.

“The situation is very grave,” Chief Cabinet Secretary Yukio Edano told reporters Tuesday. “We are doing our utmost efforts to contain the damage.”

A tsunami spawned by a magnitude-9.0 earthquake March 11 destroyed the power systems needed to cool the nuclear fuel rods in the complex, 220 kilometers northeast of Tokyo.

Since then, three of the complex’s six reactors are believed to have partially melted down, and emergency crews have grappled with everything from malfunctioning pumps to dangerous spikes in radiation that have sent workers fleeing.

Radiation seeping from the plant has made its way into produce, raw milk and even tap water as far away as Tokyo, prompting some nations to halt imports from the region. Residents within a 20-kilometer radius of the plant have been urged to leave or stay indoors.

The troubles have eclipsed Pennsylvania’s 1979 crisis at Three Mile Island, when a partial meltdown raised fears of widespread radiation release. But it is still well short of the 1986 Chernobyl disaster, which killed at least 31 people with radiation sickness, raised long-term cancer rates and spewed radiation across much of the northern hemisphere.

A series of missteps and accidents, meanwhile, have raised questions about the handling of the disaster, with the government revealing growing frustration with TEPCO.

The Yomiuri daily newspaper reported that the government was considering temporarily nationalizing the troubled nuclear plant operator, but Edano and TEPCO officials denied holding any such discussions.

The nuclear crisis has complicated the government’s ability to address the humanitarian situation facing hundreds of thousands left homeless by the twin disasters. The official number of dead surpassed 11,000 on Tuesday, police said, and the final figure is expected to top 18,000.

The urgent mission to stabilize the Fukushima plant has been fraught with setbacks.

Workers succeeded last week in reconnecting some parts of the plant to the power grid. But as they pumped water into units to cool the reactors down, they discovered pools of contaminated water in numerous spots, including the basements of several buildings and in tunnels outside them.

The contaminated water has been emitting radiation exposures more than four times the amount the government considers safe for workers and must be pumped out before electricity can be restored to the cooling system.

That has left officials struggling with two crucial but sometimes-contradictory efforts: pumping in water to keep the fuel rods cool and pumping out ― and then safely storing ― contaminated water.

Nuclear safety official Hidehiko Nishiyama called it “delicate work.” He acknowledged that cooling the reactors took precedence over concerns about leakage.

“The removal of the contaminated water is the most urgent task now, and hopefully we can adjust the amount of cooling water going in,” he said, adding that workers were building makeshift dikes with sandbags to keep contaminated water from seeping into the soil outside.

The discovery of plutonium, released from fuel rods only when temperatures are extremely high, confirms the severity of the damage, Nishiyama said.

Of the five soil samples showing plutonium, two appeared to be coming from leaking reactors while the rest were likely the result of years of nuclear tests that left trace amounts of plutonium in many places around the world, TEPCO said.

Plutonium is a heavy element that doesn’t readily combine with other elements, so it is less likely to spread than some of the lighter, more volatile radioactive materials detected around the site, such as the radioactive forms of cesium and iodine.

“The relative toxicity of plutonium is much higher than that of iodine or cesium but the chance of people getting a dose of it is much lower,” says Robert Henkin, professor emeritus of radiology at Loyola University’s Stritch School of Medicine. “Plutonium just sits there and is a nasty actor.”

When plutonium decays, it emits what is known as an alpha particle, a relatively big particle that carries a lot of energy.

When an alpha particle hits body tissue, it can damage the DNA of a cell and lead to a cancer-causing mutation.

Plutonium also breaks down very slowly, so it remains dangerously radioactive for hundreds of thousands of years.

“If you inhale it, it’s there and it stays there forever,” said Alan Lockwood, a professor of Neurology and Nuclear Medicine at the University at Buffalo and a member of the board of directors of Physicians for Social Responsibility, an advocacy group.