U.S. officials are concerned that the threat of a radiation leak has been downplayed and that wind patterns, which could carry a potential release of radioactive material outside a 20-kilometers radius towards Tokyo, have not been sufficiently accounted for.
The U.S. has dispatched a nuclear team from South Korea, but the Japanese have not responded to that offer, according to a U.S. official.
The Fukushima Daiichi plant, located about 170 miles northeast of Tokyo, was one of two run by the Tokyo Electric Power Co. whose cooling systems were damaged in the 8.9 earthquake and tsunami that hit Japan Friday.
At least two reactors at the Daiichi plant and three at the Fukushima Daini plant which located about 10 miles away had damaged cooling systems, the Associated Press reported. Officials declared states of emergency for the five reactors.
Ryohei Shiomi with Japan's Nuclear Safety Commission said earlier that a meltdown was possible at the Fukushima Daiichi plant.
Japanese TV images showed the crumbled remains of one of the plant building's walls with smoke emerging from the site.
An evacuation order was expanded from a 10-kilometer to 20-kilometers radius around the plant.
The incident comes as the level of water used to cool a nuclear reactor damaged in Friday's earthquake and tsunami dropped to an alarming level today, according to the Nuclear and Industrial Safety Agency, heightening fears of a larger nuclear disaster.
As of 11:20 a.m. local time, a part of the "fuel assembly" of fuel rods at the Fukushima Daiichi plant's No. 1 reactor was exposed above water, with a maximum exposure of about 90 centimeters.
If the fuel rods remain exposed, they will be damaged, releasing radioactivity.
"This is extremely serious," said Joseph Cirincione, president of the Ploughshares Fund and an expert on national security and international policy. "The best case at this point would still be the worst incident since Chernobyl.
"We are in uncharted territory," he said. "It is possible that this can be contained and we would have a very bad nuclear contamination event. But if the water levels continue to drop and the rods are exposed further it could lead to a core meltdown. The core would melt through the steel holding structure and plunge in a burning, molten mass into the concrete containment structure. If the structure is sound, it could contain the mass, if it has been structurally damaged, then it, too, could breech and we would have a massive, radioactive release."
About 27,000 liters of water, including water stored for firefighting, was being pumped into the reactor via makeshift pumps and other means in order to raise the water level above the reactor's nuclear fuel, NISA said.
"NISA just confirmed that the fuel may be partially melting," Dr. Tatsujiro Suzuki, vice-chairman of the Japan Atomic Energy Commission told ABC News. "The question is whether the situation is getting worse or not. It is reported that the level of water is declining (bad news) but pressure is also decreasing (good news). So, efforts to contain the event (need water) may be working. It is also stated that the amount of radioactivity is still small so that the general public does not need to be concerned at present."
But the situation at the Fukushima Daiichi reactor No. 1 was reported to be the most dire.