The temperature in reactor 4 has increased from Thursday, the official said, and the fuel rods -- which are supposed to be housed in a steel container surrounded by concrete -- are surrounded only by steel because the concrete has blown away.
Limited progress has been made in restoring power to the reactors, said the official. Reactor 2 is currently the only reactor to have a live power line connected to it and Japanese officials hope they can connect lines in more reactors over the weekend. The water pumps the U.S. sent require power to operate and will not be functional until electricity is restored.
The U.S. and Japan continue to disagree about whether there is water in cooling pools at reactors 3 and 4. Images from a Japanese helicopter flyover made it appear there was some, but the U.S. Nuclear Regulatory Commission said it is still not certain, given that the heat level remains extremely high.
In light of the disagreement Rep. Edward Markey, D-Mass., the top Democrat on the House Natural Resources Committee asked the Nuclear Regulatory Commission to provide daily reports and analysis of the situation.
The U.S. official also told ABC News that the situation remains very serious and that if everything went well over the course of the next few weeks, it would just go from being an emergency to remaining very dangerous.
Earlier today, Japanese officials raised the nuclear crisis level at the troubled plant, putting the ongoing battle to cool the damaged nuclear reactors and spent fuel ponds on par with the accident at Three Mile Island in 1979.
The rating of the crisis was raised to a 5 from a 4 on a scale used by the International Atomic Energy Agency. That's just two levels below Chernobyl, which was rated a 7, the highest possible level on the international scale set by the International Atomic Energy Agency. A 4 connotes a radiation problem with local effects; a 5 means its effects are wider and a 7 means the effects are seen, at least at some level, globally.
"It's a race against time. At a certain point, they might have to evacuate and then the whole reactor accident is in freefall," said theoretical physicist Dr. Michio Kaku. "I think the credibility of the utility [Tokyo Electric Power Co.] has melted down. ... Pictures don't lie."
The latest pictures of the plant show smoke billowing from the crippled nuclear reactors and spent fuel ponds as Japanese military and emergency crews desperately try to prevent a nuclear meltdown.
"This steam contains radioactive seasoning. ... This cloud is now going over northern Japan. It's being picked up in Tokyo now in small levels," Kaku said. "This whole area [around the plant] is a near deadly radiation field."
Out of six reactor units on Fukushima Daiichi plant, four have caught fire, exploded or suffered partial meltdowns in the past week. Water levels in the pools where used fuel rods are stored are believed to be dangerously low.
Two men working at the nuclear plant on the day the 9.0 earthquake and tsunami struck Japan have spoken out on the chaos that erupted the day of the disaster.
Minoru Yoshida, 63, told Japanese broadcaster NHK that he was on the first floor of unit 4 when the quake hit.
"The building shook sideways and the lights went out. I held onto a post because I thought the switchboard was going to collapse. I think there were about 200 workers on the first floor. However because of the dust in the air, I could not see very well and the fire drills were going off," Yoshida told NHK.
Another worker was standing between unit 3 and unit 4 dismantling construction scaffolding, NHK reported.
"I thought the building would be stronger. I would have never imagined the building would turn out the way it did," he said.
Experts say Japan never planned for the plants to handle a 9.0 earthquake followed by a tsunami.
Japan in Race Against Time in Nuclear Crisis
The problems at the Fukushima Daiichi plant have left engineers concerned containment vessels may be damaged and leaking.
Yukiya Amano, the head of the U.N's International Atomic Energy Agency, characterized the struggle to fix the problems as a race against time.
"We see it as an extremely serious accident," Amano said. "This is not something that just Japan should deal with, and people of the entire world should cooperate with Japan and the people in the disaster areas."
Despite the rise in rating, a U.S. official said that the worst-case scenario of a nuclear meltdown should never be encountered as long as efforts to contain the crisis continue.
"I am cautiously optimistic that we're progressing in that regard based on what we've seen, and the restoration of power and the efforts that they've made to add water, both, you know, from the outside and the top of these reactors," said Adm. Robert Willard, the head of U.S. Pacific Command, who has overall responsibility for the U.S. relief effort in Japan.
Willard, who briefed reporters from his headquarters in Hawaii, will travel Saturday to Japan to meet with senior military officials to see what more the U.S. can offer. The Japanese military, he said, was just beginning to ramp up its operations and only now is looking at some of the "long list" of offers of assistance made by the U.S. military.
The U.S. has already given the Japanese high-pressure pumps to keep water flowing to the reactor.
Nearly 140 additional Japanese specialist firefighters volunteered in Tokyo to help the mission on Friday. Each team member got a personal farewell from Tokyo Fire Department Chief Yuji Arai.
"We expect a lot of difficulties with the mission we have been given," he said. "I think it is really a dangerous assignment. ... The reputation of Japan and the lives of many people rest on your actions."
Across the nation, the Japanese bowed their heads in a moment of silence today for all those lost and devastated by the quake and tsunami.
Japanese Prime Minister also attempted to raise the spirits of his beleaguered nation a week after a 9.0-magnitude earthquake and tsunami killed 6,900 and left 10,354 still missing.
"We will rebuild Japan from scratch. We must all share this resolve," he said in a televised address.
It could take weeks to get enough water on the reactors to cool them adequately, according to Gregory Jaczko, chairman of the Nuclear Regulatory Commission. He called the situation "dynamic, difficult, and even tragic."
U.S. Officials 'Cautiously Optimistic' Worst-Case Scenario Can Be Avoided
Japan's chief cabinet secretary dismissed reports that Japanese officials had rejected previous offers of U.S. assistance and said Japan is "coordinating with the U.S. government as to what the U.S. can provide and what people really need.
"We have repeatedly asked for specific support, and indeed, they are responding to that," he added.
A nine-member assessment team will determine whether it makes sense to bring in a larger American force to Japan. That larger force would be the CCMRF team that ABC News' Martha Raddatz has reported is being considered for deployment to Japan.
Admiral Willard, the head of U.S. Pacific Command, has requested "a force of about 450 radiological and consequence-management experts to be available to us," he said, referring to the CCMRF. "They're on a prepare-to-deploy order. And one of the purposes of that advance team is to assess the need to call them forward."
Those forces would carry radiation detection equipment as a precaution and operate within a recommended U.S. evacuation radius, as needed.
The Japanese government maintains a mandatory evacuation order for a 12-mile radius around Fukushima Daiichi. Residents within 20 miles are told to stay indoors with windows sealed.
The United States has suggested a 50-mile evacuation radius and has offered chartered flights to take American military dependents who want to leave out of the area.
The estimated number of Americans in Japan range from 160,000 to 350,000 at any given time -- though such numbers are only guesses because Americans are not required to register with the U.S. embassy in Japan, the State Department said.
The U.S. embassy in Tokyo believed there were about 1,300 Americans in the tsunami-affected area.
The first charter flights left Japan Friday morning for authorized voluntary departures of military dependents, but the number of evacuees was relatively small.
ABC News' Akiko Fujita, Kirit Radia, Luis Martinez, Ann Compton, Michael S. James and James Hill, and The Associated Press contributed to the story.