Update at 6:39 p.m. ET: The International Atomic Energy Agency has released temperature readings for the spent fuel pools at reactors 4, 5 and 6.

Normally, the pools are kept at 25 degrees Celsius (77 degrees Fahrenheit) or or cooler. On Tuesday and Wednesday, the No. 4 reactor pool measured 84 degrees C (183 degrees F) but the agency had no reading for today.

Earlier today in Washington, U.S. NRC Chairman Gregory Jaczko told a congressional panel that all the water had boiled out of the No. 4 pool and that "extremely high" levels of radiation might thwart emergency efforts to prevent a disaster. Water boils at 100 degrees C (212 degrees F).

At Unit 5, today's temperature reading was 62.7 degrees C (144.8 degrees F), up about 2 degrees from Tuesday. Today, the Unit 6 pool measured 60 degrees C (140 degrees F), up 1.5 degrees from Tuesday.

The head of the IAEA, Yukiya Amano, plans to visit Japan on Thursday "to see the situation for myself."

Update at 6:09 p.m. ET: Among the emergency efforts at the Fukushima plant, Tokyo Electric Co. says it trying to install new power lines to restart the cooling systems that failed after Friday's magnitude 9 earthquake and resulting tsunami.

NHK TV says the plant operator hopes to run the new lines from another power plant through a makeshift switchboard Thursday, essentially creating giant jumper cables. High radiation thwarted work Wednesday.

NHK writes that TEPCO wants to finish the installation "as soon as possible after reviewing the procedures in order to keep the workers' radiation exposure to a minimum."

Update at 4:42 p.m. ET: A special police water cannon truck arrived early Thursday to spray the spent fuel rods in hopes of cooling them, Kyodo News says.

An official with Japan's Nuclear and Industrial Safety Agency said that the spent-fuel pool at the No. 3 reactor is also apparently boiling and that if cooling operations do not proceed well, the situation will ''reach a critical stage in a couple of days.''

ABC News is quoting a U.S. official as saying that if the reactors and spent fuel rods aren't controlled in 24 to 48 hours, the results "could be deadly for decades."

U.S. officials are urging Japanese officials to stop withdrawing workers because of radiation exposure and "get more people back in there."

"There is a recognition this is a suicide mission," one official said.

Japanese government sources said the U.S. military will send a high-altitude, unmanned Global Hawk reconnaissance aircraft to take images of the inside of the No. 4 reactor building. That could happen Thursday.

The building's outer walls were damaged Tuesday by what appeared to be a hydrogen explosion.

Original post: There's no more water in the pool holding spent fuel rods at the No. 4 reactor of the Fukushima Dai-ichi nuclear plant, says the head of the U.S. Nuclear Regulatory Commission, according to the Associated Press.

If all water is gone from the cooling pool, the exposed fuel rods will overheat and could possibly melt or even explode, spreading highly radioactive material.

NRC Chairman Gregory Jaczko did not say how he obtained that information.

The Japanese nuclear safety agency and the plant operator, Tokyo Electric Power Co., have denied the claim. A utility spokesman said the "condition is stable" at Unit 4.

The Union of Concerned Scientists has background on the pools. It also explains what happens if cooling in the spent fuel pools is stopped:

The radioactive particles in the fuel will continue to decay and produce additional heat. If the spent fuel cooling stops, this heat will raise the temperature of the spent fuel rods. At a high enough temperature the cladding of the rods will start to burn and produce hydrogen, which can explode. The burning of the fuel rod will damage the cladding, allowing the release of radioactive gasses that were produced by the fission reactions when the fuel was in the reactor. Further heating can cause the fuel pellets within the cladding to begin to melt, which will release larger amounts of radioactive gases into the air.

The announcement comes on the heels of the U.S. Embassy's advisory that Americans within 50 miles of the plant should evacuate as a precaution.