The State Department says the first U.S. evacuation flight has left Japan Thursday and heading to Taiwan with about 100 people on board.

Officials also warned U.S. citizens to defer all non-essential travel to any part of the country as unpredictable weather and wind conditions risked spreading radioactive contamination from Friday's magnitude 9 earthquake and subsequent tsunami.

The travel warning extends to U.S. citizens already in the country and urges them to consider leaving. The authorized departure offers voluntary evacuation to family members and dependents of U.S. personnel in Tokyo, Yokohama and Nagoya and affects some 600 people. The U.S. also urged Americans within 50 miles of the crippled Fukushima Dai-ichi nuclear plant to relocate.

Officials defended the proposed evacuation zone for American troops and citizens in Japan.

"I want to stress this is a prudent and precautionary measure to take," Gregory Jaczko, chairman of the Nuclear Regulatory Commission, told a White House briefing. The evacuation zone recommended by the U.S. is far wider than that established by Japan, which has called for a 12-mile zone and has told those within 20 miles to stay indoors.

Jaczko also said that radiation leaking from Japan's nuclear plant does not present a danger to the western U.S. or its Pacific territories at this time.

Senior State Department official Patrick Kennedy said chartered planes will be brought in to help private American citizens wishing to leave. Flights will continue for as long as necessary, and officials were advising Americans to bring food and water with them to the airport.

The U.S. military says all families at bases on Japan's Honshu Island are eligible for voluntary departure, according to Reuters. The Pentagon tells Reuters the order applies to some 20,000 dependents of military personnel. For U.S. diplomats, flights were leaving from military bases.

The U.S. sent a group of nine experts to Japan as a "consequence management response team," a senior U.S. official told Fox News. The official described them as planners, and their job is to work with the commander of U.S. Force Japan to tell him how the U.S. would potentially operate in a radiological environment.

While one group comes in, another leaves as British and American search and rescue teams will end their operations in Friday and begin to pull out of the quake-stricken country.

Japan tells the IAEA that engineers were able to lay an external grid power cable to the Unit 2 reactor, according to Reuters. The line would allow the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds.

Japanese officials are at odds over whether water dumps on the crippled Fukushima Dai-ichi nuclear plant Thursday worked as high radiation levels have been detected 19 miles away from the plant.

A spokesman for the Tokyo Electric Power Company told Japan news agency NHK that "it appears the mission was successful," while a spokesman for the Nuclear And Industrial Safety Agency says the water cannons failed in their attempt to cool the unit when the water failed to reach its target from safe distances.

Japanese military helicopters dumped water onto the crippled Fukushima Dai-ichi nuclear plant’s damaged reactors and emergency crews tried unsuccessfully to douse the reactors with water cannons.

An official from the Tokyo Electric Power Company told Japan news agency NHK "that there is a greater possibility they will be able to fill the spent fuel rod pool" for the Unit 3 reactor.

The pool requires 1,200 tons of water to be filled, but the power company official says "you only need one-third that amount to cover the spent fuel rods." Without water, there's nothing to stop the fuel rods from getting hotter and ultimately melting down.

Two twin-rotor CH-47 Chinooks from the Japanese Self-Defense Forces were used in the operation, working to drop seawater on the No. 3 and No. 4 reactors.

The choppers dumped at least four loads on the reactor in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.

Chopper crews flew missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 7,500 liters of water.

A spokesman for the nuclear plant called it a "very severe operation," according to Sky News.

An International Atomic Energy Agency spokesman says the situation remains "very serious, but there has been no significant worsening since yesterday (Wednesday)."

Graham Andrew added that reactors 1, 2, and 3 "appear to be relatively stable, while Unit 4 remains a "major safety concern."

Japan's science ministry tells NHK that high levels of radiation have been detected 19 miles from the nuclear plant and the government has instructed residents to stay indoors.

Experts say exposure to that level of radiation for six hours would result in the maximum level considered safe for one year.

The White House said it would cooperate closely with Japan during the recovery period, and President Obama spoke with Japanese Prime Minister Naoto Kan by phone on Wednesday evening.

They discussed Japan's efforts to recover from last week's devastating earthquake and tsunami, and the nuclear crisis at the Fukushima Dai-chi plant. Obama promised Kan that the U.S. would offer constant support for its close friend and ally, and "expressed his extraordinary admiration for the character and resolve of the Japanese people," the White House said.

The International Atomic Energy Agency confirmed Wednesday that three reactors had partially melted down. Yukiya Amano, the head of the nuclear watchdog agency, says he plans on going to Japan as soon as possible.

When asked if events were out of control, he answered: "It is difficult to say."

Meanwhile, 50 employees at the Fukushima Dai-ichi nuclear plant -- dubbed the Fukushima 50 -- have put their health and well-being on the line as they try to prevent a total nuclear meltdown with conditions worsening at the plant.

The International Atomic Energy Agency reports that 20 of the workers have suffered from radiological contamination, 19 have been injured, while two others are missing.

The government has now raised the maximum radiation dose allowed for nuclear workers to 250 millisieverts from 100 millisieverts.

It is not known how much radiation has leaked from the crippled nuclear plant because the computer system that forecasts the spread of radioactivity has not been working due to malfunctioning monitoring posts, according to NHK.

A U.S. nuclear expert said he feared the worst.

"It's more of a surrender," said David Lochbaum, a nuclear engineer who now heads the nuclear safety program for the Union of Concerned Scientists, an activist group. "It's not like you wait 10 days and the radiation goes away. In that 10 days things are going to get worse."

"It's basically a sign that there's nothing left to do but throw in the towel," Lochbaum said.

There are six reactors at the plant. Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions.

Units 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel -- either inside the reactors or in storage ponds -- that need to be kept cool.

"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."

The grim search for survivors continues in areas the earthquake and tsunami as the mayor of a northeastern city said his town has been wiped out.

Mayor Kameyama Hiroshi told Kyodo News that 10,000 people remain missing in the city of Ishinomaki. Before the tsunami wiped out the coastal town, 164,000 people were living there.

Japan's National Police Agency says more than 12,000 people are reported missing or dead. More than 5,600 people are officially listed as dead, but officials believe the toll will climb to well over 10,000.

The country's Defense Ministry tells Japan news agency NHK that more than 25,000 have been rescued and another 23,000 are still believed to be stranded on islets near the coast.

Nearly a week after the disaster, police said more than 452,000 people were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short. Both victims and aid workers appealed for more help.

"There is enough food, but no fuel or gasoline," said Yuko Niuma, 46, as she stood looking out over Ofunato harbor, where trawlers were flipped on their sides.

Along the tsunami-savaged coast, people must stand in line for food, gasoline and kerosene to heat their homes. In the town of Kesennuma, they lined up to get into a supermarket after a delivery of key supplies, such as instant rice packets and diapers.

Each person was only allowed to buy 10 items, NHK television reported.

With diapers hard to find in many areas, an NHK program broadcast a how-to session on fashioning a diaper from a plastic shopping bag and a towel.

In an extremely rare address to the nation Thursday, Emperor Akihito expressed condolences and urged Japan not to give up.

"It is important that each of us shares the difficult days that lie ahead," said Akihito, 77, a figure deeply respected across the country. "I pray that we will all take care of each other and overcome this tragedy."

The Pentagon said U.S. troops working on relief missions can within 50 miles to the plant with approval. Spokesman Col. David Lapan said the U.S. would review requests from the Japanese for assistance that would require troops to move within that radius, though no approval for such movement had been given since the stricter guidelines were enacted.

The Pentagon said troops are receiving anti-radiation pills before missions to areas where radiation exposure is likely.

With the arrival of three more ships to the massive humanitarian mission, there were 17,000 sailors and Marines afloat on 14 vessels in waters off Japan. Several thousand Army and Air Force service members already stationed at U.S. bases in Japan have also been mobilized for the relief efforts.

The Associated Press and NewsCore contributed to this report.