TOKYO — U.S. military manpower and equipment started arriving in Japan on Saturday to assist in rescue and humanitarian operations, as the death toll from Friday’s 8.9-magnitude earthquake and powerful tsunami mounted along with concerns about the safety of nuclear power facilities in northern Japan.

Officials said late Saturday about 600 had been killed, but The Associated Press reported that local media outlets were estimating the number of dead will likely climb to more than 1,300.

While aftershocks continued to rumble across the country Saturday, and the search for bodies of the dead and the missing continued, the most pressing concern of the day was that fate of a couple of Japan’s nuclear power plants.

Officials declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability, including the Fukushima Dai-ichi plant 20 miles from Iwaki, where an explosion Saturday prompted Prime Minister Naoto Kan to order everyone living within a 12-mile radius of the facility to evacuate.

More than 125 aftershocks have occurred, many of them above magnitude 6.0, which alone would be considered strong. AP reported that more than 215,000 people were living in 1,350 temporary shelters in five prefectures, or states. Since the quake, more than 1 million households have not had water, mostly in the northeast part of the country.

The transport ministry said all highways from Tokyo leading to quake-hit areas were closed, except for emergency vehicles. Mobile communications were spotty and calls to the devastated areas were going unanswered.

Despite the widespread death and destruction, officials said the U.S. military in Japan escaped relatively unscathed with no reported deaths or injuries, nor significant damage from the disaster. The hardest hit U.S. facility was Misawa Air Base in northern Japan, where most residents remained without heat or electricity Saturday and many scrambled to stock up on supplies at the commissary before nightfall. At the Potter Fitness Center, members of the 35th Force Support Squadron set up nearly 200 cots and sleeping bags.

U.S. ships and aircraft began heading to mainland Japan on Saturday following a request for help from the Japanese government.

The USS Ronald Reagan carrier strike group is expected to arrive Sunday along the east coast of Japan’s main Honshu island, where it will support relief efforts in areas hit hardest by the earthquake, 7th Fleet spokesman Cmdr. Jeff Davis said.

The Reagan was en route to South Korea, where it was scheduled to participate in a joint South Korean-U.S. military exercise this week.

The Reagan “is prepared to serve as an afloat platform for refueling Japan Self Defense Force and other helicopters involved in rescue and recovery efforts ashore,” Davis wrote in an e-mail.

Meanwhile, U.S. aircraft from Okinawa filled with medical and humanitarian relief supplies were scheduled to fly Saturday afternoon to Marine Corps Air Station Iwakuni, where they would be pre-positioned to respond quickly to the devastated regions.

Prime Minister Kan also ordered 50,000 Japanese troops to take part in rescue operations. Japan had received offers of help from 50 countries, Kan said.

“We need to put our utmost effort into saving survivors and those who were isolated today,” Kan said during an emergency meeting Saturday morning. “Today is a very important day on how much rescue efforts will move forward.”

Japan’s Ministry of Defense said in a news release that it had asked U.S. Forces Japan to transport approximately 900 Japanese troops and 250 vehicles to disaster-hit areas. The U.S. Navy’s 7th Fleet and Japanese naval forces were making plans for a joint rescue operation, according to an MOD news release.

U.S. military officials in South Korea, Japan and at the U.S. Pacific Command in Hawaii could not say Saturday whether U.S. troops stationed elsewhere in the Pacific region would be asked to participate in rescue efforts. However, U.S. Secretary of Defense Robert Gates said Friday that the U.S. would help “in any way we possibly can.”

Travel disrupted

Transportation on the ground and in the air is still a major problem.

While much of Tokyo seemed to return to business as usual Saturday, outbound traffic from Tokyo headed toward the devastated Miyagi Prefecture on Route 4 backed up for hours Saturday afternoon and into the evening.

Japanese authorities closed portions of the Tohoku Expressway running parallel to Route 4 and were only allowing emergency vehicles to enter the highway as help continued pouring into Miyagi.

News of an explosion at a nuclear power plant in Fukushima, roughly 75 miles south of Sendai, began hitting the radio late Saturday afternoon. Electronic signs on the roads warned drivers that only “designated” vehicles would be allowed to continue toward Fukushima.

At Yokota Air Base, 537 passengers from two commercial airplanes happily loaded onto buses Saturday afternoon and continued on to Narita International Airport after they were detoured to the base in the wake of the earthquake.

Passengers from Delta Flight 295 and Delta 647 were two of 11 commercial planes diverted from Narita in the aftermath of the quake. The two were the only flights unable to depart Friday, leaving passengers stranded overnight at the base.

Passenger Richard Emerson was anxious about continuing his trip.

“It’s been an adventure,” he said with a laugh. “It’s the start of a seven-week vacation for us — a hell of way to start it.”

Emerson said his heart went out to those more impacted by the disaster.

“This is such a tragic thing for the Japanese up there and all of Japan,” he said.

With communications still spotty in many areas of Japan, Internet social media and news sites were utilized by people checking on the welfare of family and friends.

The U.S. State Department set up e-mail addresses to help the effort. Americans in Japan who need help, or people seeking information about a loved one in Japan, can e-mail japanemergencyusc@state.gov. Americans outside Japan but in tsunami-affected areas who need help, or people seeking information about an American in affected areas outside Japan, can e-mail pacifictsunamiusc@state.gov. An information line also has been set up at 1-888-407-4747.