Tokyo -- Water found in a tunnel at the Fukushima Daiichi nuclear plant has alarmingly high radiation readings, officials said Monday, adding that it is unclear how or why the tainted water got out of the building.

The water at the plant is emitting more than 1,000 millisieverts per hour of radioactivity -- a level the plant's owner had said is at least 100,000 times normal levels for coolants inside a nuclear reactor.

It was in a tunnel that contains electrical cables and is connected to the No. 2 reactor's turbine building, an official with the Tokyo Electric Power Co. said. The measurements were taken Monday afternoon.

Earlier, officials had announced that 1,000 millisieverts per hour of radiation was emanating from water pooling inside the No. 2 unit turbine building's basement.

The officials said they don't know how or why the contaminated water got out of the building and into the tunnel, or if it might have spilled out and seeped into the Pacific Ocean.

The measurement is more than 330 times the dose an average person in a developed country receives per year, and four times the top dose Japan's health ministry has set for emergency workers struggling to control the further emission of radioactive material from the damaged plant.

"Is the water overflowing or not?" Hidehiko Nishiyama, an official with Japan's nuclear and industrial safety agency, said Monday evening. "Right now, it is not known."

On Monday, the agency reported that seawater tested at an offshore monitoring post near the plant's Nos. 5 and 6 nuclear reactors had radiation levels 1,150 times the control level.

This is north of the discharge canal for the Nos. 1, 2, 3 and 4 reactors, where a reading of 1,850 times normal was recorded Sunday.

Readings of atmospheric radiation taken Monday near the No. 2 unit's tunnel came in between 100 and 200 millisieverts, according to Nishiyama.

This figure is much higher than the measurements outside of the plant's gates, but still less than a 400-millisieverts-per-hour reading measured between Units 3 and 4 on March 15.

Authorities also worked Monday to test water and airborne radiation readings in tunnels coming from the Nos. 1 and 3 reactor's turbine buildings, where highly radioactive water was also found in those structures' respective basements.

Both the water and air around the No. 1 unit's tunnel measured 0.4 millisieverts per hour of radiation, Nishiyama said.

The atmospheric radiation reading outside of the No. 3 unit's tunnel was 0.8 millisieverts, but debris and damage caused by the March 11 quake and subsequent tsunami prevented authorities from getting a reliable reading of radioactivity in the water in that tunnel.

Three pumps are being used to pump water from the basement of the No. 1 turbine building. But Nishiyama noted that there is no place to put water pooled in the No. 2 building's basement.

The plan is to extract the water using what he called a condenser. But that apparatus is "almost full," as are several storage tanks nearby. A similar challenge is holding up the removal of collected water in the No. 3 unit's turbine building basement.

"So we will first have to empty some of the tanks," he said, adding later only that the tainted water needs to be removed "as soon as possible." "Once that process is over, the puddle would be removed."

Tokyo Electric had come under fire Sunday for its erroneous report on the radioactivity of the water inside the No. 2 building -- first saying it was 10 million times normal, before correcting itself and saying that the reading was 100,000 times normal.

Japanese Chief Cabinet Secretary Yukio Edano said Monday that the plant's owner cited fatigue among its workers as a reason for the error.

"However, measurement of radioactivity is vital for the safety of the workers there," Edano told reporters. "So such a mistake is not something that should be forgiven or acceptable."

Later Monday, Edano noted -- as experts have said previously -- he has received a report that the No. 2 unit's containment building, which houses and protects the reactor core, "is damaged and water is leaking."

But he added that he hasn't gotten a report about the condition of the unit's pressure vessel, which is inside of the larger containment building.

While high levels of radiation of water in the Nos. 2 and 3 turbine buildings -- and to a lesser extent in the No. 1 unit -- have been the chief focus of late, they aren't the only problems at the facility, which is 240 kilometers (150 miles) north of Tokyo.

Most of the concerns have centered around the Nos. 1, 2 and 3 units. They were the only ones operating -- and with active fuel rods in their reactor cores -- on March 11, when the quake and tsunami knocked out backup generators that ran their coolant systems and damaged water pumps at the plant, forcing workers to scramble to prevent a meltdown.

Despite reduced alarms in recent days, Nishiyama noted Monday that the temperature is rising inside the No. 1 reactor.

The spike in heat at the No. 1 unit could be a sign that nuclear fuel rods are overheating. If those fuel rods are fully or partially exposed, that could lead to a buildup of pressure that could cause an explosion or the release of more radiation into the air, soil or water.

To address this issue, the flow of fresh water into the reactor core will be further adjusted, the nuclear safety official said.

That water is being directed via a fire truck and temporary electricity-driven pump with a more permanent power generator likely in place by Tuesday.

Authorities plan to also get distinct power sources for the cooling systems for units Nos. 2 and 3. Fresh water is being pumped into those two reactor cores, also using a fire truck and temporary electricity-powered pumps.

Between Monday and Tuesday, authorities hope to switch from using seawater to fresh water in these three unit's spent nuclear fuel pools, where some fuel rods are also located.

Besides covering and keeping nuclear fuel cool, the fresh water will help flush out salt so the cooling systems can operate better.