SENDAI, Japan » Japan launched a massive, military-led rescue operation today after a giant quake and tsunami killed hundreds of people and turned the northeastern coast into a swampy wasteland as authorities braced for possible meltdowns by nuclear reactors.

Prime Minister Naoto Kan said he is sending 50,000 troops for the rescue and recovery efforts following yesterday’s 8.9-magnitude quake that unleashed one of the greatest disasters Japan has witnessed: a 23-foot tsunami that washed far inland over fields, smashing towns, airports and highways in its way.

The official death toll exceeded 500, while 740 people were missing and 1,128 injured. In addition, police said between 200 and 300 bodies were found along the coast in Sendai, the biggest city in the area near the quake’s epicenter. An untold number of bodies were also believed to be lying in the rubble and debris. Rescue workers had yet to reach the hardest-hit areas.

Adding to the worries was the damage at the Fukushima Daiichi nuclear power plant, where two reactors had lost cooling ability.

Officials issued broad evacuation orders today for people living in the vicinity of that nuclear power plant and another that had experienced breakdowns in their cooling systems as a result of the earthquake, and they warned that small amounts of radiation could leak from both plants.

More than 215,000 people were living in 1,350 temporary shelters in five prefectures, the national police agency said. Since the quake, more than 1 million households, mostly concentrated in the northeast, have not had water.

Kan said 190 military planes and 25 ships were sent to the area.

"Most of the houses along the coastline were washed away, and fire broke out there," he said after inspecting the quake area in a helicopter. "I realized the extremely serious damage the tsunami caused."

Meanwhile, huge earthquakes rocked northeastern Japan today.

The U.S. Geological Survey said a strong earthquake struck just before noon in the sea in virtually the same place where yesterday’s quake hit.

Today’s magnitude-6.8 quake was followed by a series of temblors originating from the same area, the USGS said. It was not immediately known whether the new quakes caused any more damage. All were part of the more than 125 aftershocks since yesterday’s massive quake, the strongest to hit Japan since officials began keeping records in the late 1800s.

It ranked as the fifth-largest earthquake in the world since 1900 and was nearly 8,000 times stronger than one that devastated Christchurch, New Zealand, last month, scientists said.

Smashed cars and small airplanes were jumbled up against buildings near the local airport in Sendai, several miles from the shore. Felled trees and wooden debris lay everywhere as rescue workers coasted on boats through murky waters around flooded structures, nosing their way through a sea of detritus.

"The flood came in from behind the store and swept around both sides. Cars were flowing right by," said Wakio Fushima, who owns a convenience store in this northern coastal city of just more than 1 million people, 80 miles from the quake’s epicenter.

Basic commodities were at a premium. Hundreds lined up outside of supermarkets, and gas stations were swamped with cars. The situation was similar in scores of other towns and cities along the 1,300-mile-long eastern coastline hit by the tsunami.

In Sendai, as in many areas of the northeast, cell phone service was down, making it hard for people to communicate with loved ones.

"I’m waiting for my son to come here. But I cannot tell him he should come over here because mobile phones aren’t working," a woman in her 70s at a shelter told Japanese TV in the town of Rikuzentakata, which appeared to be largely destroyed by the tsunami.

"My husband is missing," she said. "Tsunami water was rising to my knees, and I told him I would go first. He is not here yet."

The tsunami swept inland about six miles, and beyond that most buildings appeared undamaged.

TV footage showed several people standing on the roof of a three-story building in Miyagi prefecture, surrounded by mud. A man waved a big white flag and a woman lifted two pink umbrellas to signal for help.

Elsewhere, aerial footage showed military helicopters lifting people on rescue tethers from rooftops and partially submerged buildings surrounded by water and debris. At one school a large white "SOS" had been spelled out in English.

President Barack Obama pledged U.S. assistance following what he called a potentially "catastrophic" disaster. He said one U.S. aircraft carrier was already in Japan and that a second was on its way. A U.S. ship was also heading to the Mariana Islands to assist as needed, he said.

Most trains in Tokyo started running again today after the city had been brought to a near standstill the day before.

Japan’s worst previous quake was a magnitude-8.3 temblor in Kanto that killed 143,000 people in 1923, according to the USGS. A magnitude-7.2 quake in Kobe killed 6,400 people in 1995.

Japan lies on the "Ring of Fire" — an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world’s quakes occur.