Japanese Self-Defense Force members are carrying out the largest search yet for 12,000 people who went missing after the March 11 earthquake and tsunami.

The U.S. military and the Japanese police are also taking part in the search, which will involve 25,000 people, 90 aircraft and 50 ships over the next two days.

The search is covering inland and coastal areas of Iwate, Miyagi and Fukushima prefectures, along with the offshore waters. It will also include areas within 30 kilometres of the damaged Fukushima Daiichi nuclear plant that were not previously covered.

Official figures show the quake and tsunami, along with the aftershocks on April 7 and 11, have killed about 14,300 people.

An activist wearing an anti-nuclear mask takes part in a rally against nuclear power plants in Tokyo on Sunday. Kim Kyung-Hoon/Reuters

In the town of Shichigahamamachi on Monday, a line of about two dozen Japanese soldiers walked in unison across soggy earth and muddy pools, plunging their poles about 60 centimetres into the muck to ensure they don't miss any bodies buried below.

The search focused on a marsh drained in recent weeks by members of the army's 22nd infantry regiment using special pump trucks.

Several dozen other soldiers cleared mountains of rubble by hand from a waterfront neighbourhood filled with gutted and teetering houses. Four people in the neighbourhood were missing, said 67-year-old Sannojo Watanabe.

"That was my house right there," he said, pointing to a foundation with nothing atop it.

He surveyed the neighbourhood: "There's nothing left here."

In all, 370 troops from the regiment were searching for a dozen people still missing from Shichigahamamachi. The regiment had been searching the area with a far smaller contingent, but tripled the number of troops it was using for the two-day intense search, said Col. Akira Kun itomo, the regimental commander.

The search is far more difficult than that for earthquake victims, who would mostly be buried in the rubble, said Michihiro Ose, a spokesman for the regiment. The tsunami could have left the victims anywhere, or even pulled them out to sea.

"We just don't know where the bodies are," he said.

Bodies likely unrecognizable

Bodies found so many weeks after the disaster are likely to be unrecognizable, black and swollen, Ose said.

"We wouldn't even know if they would be male or female," he said.

The military's first intense sweep for bodies uncovered 339, while its second turned up 99 more, Defence Ministry spokesman Norikazu Muratani said. By Monday evening, searchers had found 38 more.

After the 2004 Indian Ocean tsunami, bodies turned up along the Indonesian coast for several months afterward as people cleared debris in reconstruction efforts. However, 37,000 of the 164,000 people who died in Indonesia simply disappeared, their bodies presumably washed out to sea.

Last week, two undersea robots provided by the non-profit International Rescue Systems Institute conducted five-day searches in waters off Japan's northeastern coast near three tsunami-hit towns.

The robots found cars, homes and other wreckage in the sea, but no bodies, said Mika Murata, an official with the institute.

The Japanese government has come under criticism for its response to the quake, tsunami and subsequent nuclear disaster, with some members of the country's opposition urging Prime Minister Naoto Kan to resign.

Prime Minister Naoto Kan answers a lawmaker's question at a budget committee meeting in the upper house of parliament in Tokyo on Monday. Kim Kyung-Hoon/Reuters

On Monday, Kan stressed to a sometimes hostile parliament that his government was doing everything it could to gain control of the radiation leaks at the nuclear plant, which has prompted the government to evacuate residents from a 20-kilometre area around the crippled reactors.

"The nuclear accident is still ongoing," he said. "The top priority right now is to stabilize it."

In Fukushima prefecture, the government has launched an operation to euthanize some of the farm animals left in the no-entry zone.

Six officials, including veterinarians, entered the area on Monday, the first day of the mission. There are more than 370 livestock farms in the no-go zone, with 4,000 cattle, 30,000 pigs, 630,000 chickens, and 100 horses. But many of these animals have died or are facing starvation since their owners evacuated the area. Some of them remain outdoors.

The plan is to kill the weakened animals and disinfect carcasses. A veterinarian says animals found alive will get medical examinations. The prefecture says it will not kill any animals unless their owners agree because there is no law stipulating what should be done in such a situation.

People are concerned about pets in the exclusion zone, too.

An animal welfare coalition in Japan is hoping the government will allow it to continue to search for pets that have been abandoned.

With files from CBC's Craig Dale