Get up-to-the-minute developments at CNN's live blog on the disaster in Japan.
Washington (CNN) -- The death toll from the monster 9.0-magnitude earthquake and massive tsunami that hit Japan climbed past 6,400 Friday as search teams continued to comb through the rubble.
Japan's National Police agency said 6,406 people were confirmed dead and 10,259 were reported missing as of 9 a.m. Friday (8 p.m. Thursday ET).
Meanwhile Japanese authorities vowed Friday to keep dousing water on a troubled nuclear reactor, with its owner saying that earlier attempts have been "somewhat effective" in addressing radiation concerns.
Still, the Fukushima Daiichi complex of six nuclear reactors remained a danger. Radiation levels there Thursday morning measured nearly 3.8 millisieverts per hour -- more than a typical resident of a developed country receives in a year -- Tokyo Electric said. A company official noted on Friday afternoon that radiation levels at the plant's west gate, measured at .26 to .27 millisieverts, have been fairly stable over a recent 12-hour span.
U.S. President Barack Obama said Thursday that he was "heartbroken" over events in Japan but was offering resources to help the Japanese recover as well as to keep U.S. citizens out of harm's way.
In a statement at the White House, Obama noted that the Nuclear Regulatory Commission on Wednesday called for U.S. citizens to evacuate to beyond a 50-mile radius of the stricken plants in northeastern Japan.
That figure exceeds the 12.4-mile evacuation radius and 18.6-mile shelter radius called for by the Japanese government. Obama said the stricter U.S. recommendation was "based upon a careful scientific evaluation."
He also noted that the government has authorized the voluntary departure of family and dependents of U.S. officials in northeast Japan, and he called on all U.S. citizens in Japan "to carefully monitor the situation."
The crisis poses little risk to people in the United States, he said.
"Whether it's the West Coast, Hawaii, Alaska or U.S. territories in the Pacific, we do not expect harmful levels of radiation," he said. "That's the judgment of our Nuclear Regulatory Commission and many other experts."
He added that public health experts do not recommend that people in the United States take precautionary measures beyond staying informed.
Though U.S. nuclear plants "have undergone exhaustive study and been declared safe for any number of extreme contingencies," Obama said that, in light of the disaster, he has asked the Nuclear Regulatory Commission to carry out a comprehensive review of the plants.
Meanwhile, the United States is "working aggressively" to offer support, with search-and-rescue teams and disaster-response teams and members of the U.S. military working in Japan. "To date, we have flown hundreds of missions to support the recovery efforts and distributed thousands of pounds of food and water."
He added, "We're sharing with them expertise, equipment and technology so that the courageous responders on the team have the benefit of American teamwork and support."
Earlier, during a visit to the Japanese Embassy, Obama said he was "heartbroken" over the events.
"I'm going to be making a statement, communicate how heartbroken we are of this tragedy," he said at the Embassy, where he met with Ambassador Ichiro Fujisaki and signed a book of condolences.
"Please know that America will stand by one of its greatest allies during this time of need," he wrote. "Because of the strength and wisdom of its peoples, we know that Japan will recover, and indeed will emerge stronger than ever."
Outside the embassy, flags hung at half-staff.
Washington's attention to the disaster came as thousands of Japanese settled, perhaps indefinitely, into shelters as the government scrambled to avert a wider nuclear disaster and foreigners sought to get out of the nation.
Foreign nationals formed long lines at the Tokyo immigration office for permits to temporarily leave, and the U.S. Defense Department authorized the voluntary departure of some relatives of servicemembers stationed in Japan.
The State Department said Americans may still be inside the 50-mile evacuation zone. "I would be surprised if we don't (have Americans in the zone) because we have literally thousands of names of Americans who reside in Japan," Deputy Assistant Secretary of State James Pettit said Thursday. "It would be hard to believe some of them do not live within that zone."
The State Department was sending a fleet of 14 buses to Sendai, Japan -- north of the evacuation zone -- to evacuate perhaps as many as 600 Americans who may still be in one of the areas hardest hit by the earthquake and tusunami and having difficulty traveling because of road damage.
"Those buses are en route, the consular teams are up in that area. They are getting out the word where the rallying points are," Undersecretary of State Patrick Kennedy said Thursday. He said the Americans were private American citizens either living or touring in Japan at the time of the disaster.
Kennedy said there were conflicting numbers of how many Americans were in Japan when the earthquake and tsunami hit, with some estimates as high as 550,000.
"They are all guesses," Pettit said.
The potential number of U.S. military evacuees is "in the thousands," Pentagon spokesman Col. Dave Lapan said, adding that the authorization applies only to relatives living on the island of Honshu -- site of the damaged Fukushima Daiichi nuclear power plant -- and will not cover those who are simply visiting.
The United States has also authorized $35 million in humanitarian assistance for Japan, Lapan said, and the U.S. military is sending a nine-member team specializing in responding to nuclear hazards to advise the Japanese government. The team can assess radiation damage and advise on cleanup, among other capabilities.
In the hardest-hit parts of the country, thousands of people, many of them frail and elderly, settled into shelters not knowing when they might be able to leave.
They busied themselves with the ordinary: standing in line for lunch, or arranging the few belongings they salvaged before water washed away their homes.
The government tried furiously to cool nuclear reactors in its latest bid to avert a wider disaster. Helicopters, fire trucks and police water cannon dumped or shot water on the No. 3 unit at the Fukushima Daiichi nuclear power plant.
Stocks dropped Thursday in Japan. The Nikkei 225 index, a measure of stocks traded in Tokyo, was down 454 points, or 5%, before rebounding and ending a nervous trading day down 131 points, or 1.4%.
On Thursday morning, Japanese forces made four helicopter passes in about a 20-minute span, dropping 7.5 tons of seawater each time on the facility's No. 3 unit in an attempt to cool its overheated spent fuel pool. Video of the operation on public broadcaster NHK showed that only one of the loads appeared to drop directly on the building. Experts say that steam rising from that pool, which holds at least partially exposed fuel rods, may be releasing radiation into the atmosphere.
But hours later, the Tokyo Electric Power Company, which runs the plant, said the efforts would continue.
A Japanese government spokesman, Noriyuki Shikata, tried to allay fears of an imminent meltdown. "We have not seen a major breach of containment" at any of the plant's troubled nuclear reactors, he said.
A meltdown occurs when nuclear fuel rods cannot be cooled and the nuclear core melts. In the worst-case scenario, the fuel can spill out of the containment unit and spread radioactivity through the air and water.
That, public health officials say, can cause both immediate and long-term health problems, including radiation poisoning and cancer.
Authorities also have banned flights over the area.
CNN's Charley Keyes, Kyung Lah, Larry Shaughnessy, Gary Tuchman, Stan Grant, Brian Walker, Holly Yan, Ben Rooney, Jessica Ellis and Sean Morris and journalist Diego Laje contributed to this report.