Rescue and recovery efforts after the nuclear disaster in Japan are being stymied by a nearly overwhelming array of obstacles, as government and aid groups struggle with the physical devastation of last week's earthquake and tsunami, the specter of radiation dangers and harsh weather conditions.

"The huge challenge for the aid workers on the ground is just the operating conditions they are dealing with," said Kirsten Mildren, a spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs. "Search and rescue workers are saying they've never seen anything like this."

Mildren said the tsunami that followed last week's magnitude 9.0 earthquake "took everything in its path. … The level of destruction is just monumental and you've still got flooded areas, and now on top of that you've got this rain and this snow."

The need in Japan is extreme, the United Nations reported. The 450,000 refugees crowded into 2,444 shelters don't begin to tell the story: About 1.6 million households are without water in 12 prefectures. Temperatures are below freezing in much of the area. Anxiety is rising over radiation leaks from the damaged Fukushima Dai-ichi nuclear reactors.

Children sleeping on cardboard
"Our staff in Sendai went to one of the evacuation centers and said that kids were sleeping on the ground in near-freezing temperatures on nothing but pieces of cardboard because there were no beds and they had no blankets, they had no warm clothing," said Casey Calamusa, a spokesman for World Vision, an international humanitarian organization that is working in Tome, north of Sendai.

Emi Sakai, 28, of Perth, Australia, said in a telephone interview from Tokyo that her sister had been unable to evacuate from her home about 25 miles from the Fukushima plant because there wasn't enough fuel and her baby boy had a high fever.

"She was almost crying," said Sakai, who is visiting her parents. "… There's nothing they can do. They just want to get out, but there's no way to get out — the flight is fully booked, and there is no gas to come back here.

"We still haven't lost hope yet, but it's really a tough situation, honestly," said Sakai, adding that she planned to try to help her sister and her family escape by using an online tool that gives real-time data about which gas stations have fuel and how long the wait is for it.

"Everyone is so depressed really and the TV news is heartbreaking."

Four big obstacles
Government and aid organizations identified four major obstacles that were combining to serve as nearly insurmountable roadblocks:

• Physical destruction "is just monumental,"  Mildren said.

Francis Markus, a spokesman for the Red Cross in Japan, said the country had entered "uncharted territory" because of damage unparalleled in living memory.

"They haven't experienced a disaster of this kind," Markus said. Japanese are "used to having very destructive earthquakes, but the damage that was wrought by the tsunami is much more than the earthquake in this case."

• Lack of fuel, created by the difficulty of getting supplies to the neediest areas and aggravated by hoarding, is "throwing a spanner into everything," Markus said.

"In many places, the shops are empty and there's nothing to buy because there is no fuel to deliver it," he said.

Mildren, the U.N. spokeswoman, said search and rescue teams were able to get around largely only on foot because "you've got a fuel shortage and you've got a lack of transport and cars up there."

Yukio Edano, Japan's chief cabinet secretary, urged consumers elsewhere in the country to "stop panic buying of fuel" to help free up desperately needed supplies.

International fuel assistance began taking shape Thursday, when South Korea said it would redirect some of its liquefied natural gas products to its neighbor. A spokeswoman for China's Foreign Ministry said China would ship 3 million gallons of gas and an equal amount of diesel oil at the Japanese government's request.

S Oil Corp. of South Korea said it would also boost its deliveries of oil products by more than 29 million gallons.

• Harsh winter weather is complicating an already difficult relief effort, making it so "you can't get to certain areas," Mildren said.

U.S. military officials said relief supply missions flying from the USS Ronald Reagan off the coast have been were curtailed because of wind and snow.

The USS Constellation, based in San Diego, was able to fly only three missions Wednesday, far fewer than had been expected, NBC station KNSD of San Diego reported.

Overall, the Chancellorsville and two other ships attached to the USS Ronald Reagan carrier group off the Japanese coast have been able to deliver only 8 tons of bottled water, cereal, milk, juice, fruit, medical supplies, clothing and blankets, said Thom Moore, the vessel's command master chief.

• Radiation fears have not only triggered a no-fly zone around Fukushima, further complicating transportation, but are also forcing rescue workers to take extra precautions that slow deliveries even more.

"Obviously, people have to travel a certain route to get up to the north because if they're in Tokyo, they have to go around the no-fly zone," said Mildren of the U.N. humanitarian affairs office.

Cmdr. Joe Cahill, commanding officer of the USS Preble, one of the ships attached to the Ronald Reagan group, said that a helicopter assigned to the Preble tested positive for radioactivity after apparently getting too close to Fukushima, but that the contamination was cleaned up. The crew is "safe and healthy," Cahill said, but the relief mission is having to take extra measures to monitor radiation levels.

The U.S. military said it was sending a stockpile of potassium iodide tablets to Sagami, Japan, to treat U.S. military service members and civilians who have been exposed.

Americans not giving as much
To make matters worse, some contributions to relief agencies are trickling in more slowly than they have in previous disasters, said Una Okonkwo Osili, director of research at the Center on Philanthropy at Indiana University.

Osili said that as of Thursday, six days into the relief effort, Americans had contributed about $66 million to aid organizations operating in Japan — barely a quarter of the $228 million they had donated six days after last year's earthquake in Haiti.

The faltering U.S. economy plays a role in Americans' reduced giving, but so do the insular nature and relative wealth of Japanese society, Osili said in an interview with Tamron Hall of MSNBC TV.

Numerous nonprofit groups were already in Haiti working on long-term economic development when the earthquake hit near Port-au-Prince on Jan. 12, 2010. They needed only to shift resources and personnel already on the ground to focus on disaster relief.

By contrast, few Western nonprofits were working in Japan before last month. Simply getting there and setting up facilities "makes it difficult for them to start relief operations and then to raise money," she said.

Another hurdle is the lack of clear guidance about the nature and scope of the need.

"The uncertainty's probably a bigger factor than we realize," Osili said. "In this case, there's a lack of information about how to support the Japanese people."

John Schoen and Al Stirrett of msnbc.com and NBC station KNSD of San Diego contributed to this report.