TOKYO: Japan's government said Sunday it expects the economic impact of Friday's huge 8.9-magnitude quake, and the devastating tsunami that followed, to be “considerable”.Chief Cabinet Secretary Yukio Edano said the government would hold a meeting Sunday to assess economic damage from the disaster.

“The quake is expected to have considerable impact on a wide range of our country's economic activities,” he said.

Analysts say it will take weeks to get a firm idea of the extent of the damage inflicted on the nation and its economy, as the destruction wrought by the biggest ever quake to strike Japan continues to emerge.

The government has said at least 1,000 people were believed to have lost their lives, but the police chief in badly hit Miyagi prefecture said the death toll there alone is certain to exceed 10,000.

The quake and tsunami have damaged or closed down key ports. Some airports shut in the immediate aftermath have since reopened, but transport infrastructure has been crippled along parts of the northeastern coast.

Many top Japanese firms have said they are suspending operations.

Japanese officials were struggling with a growing nuclear crisis and the threat of multiple meltdowns, as more than 170,000 people were evacuated from the quake and tsunami-savaged northeastern coast.

A partial meltdown was already likely under way at one nuclear reactor, a top official said, and operators were frantically trying to keep temperatures down at the power plant's other units and prevent the disaster from growing even worse.

Chief Cabinet Secretary Yukio Edano said Sunday that a hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, the reactor that could be melting down. That would follow a blast the day before in the power plant's Unit 1, as operators attempted to prevent a meltdown by injecting sea water into it.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."

More than 170,000 people had been evacuated as a precaution, though Edano said the radioactivity released into the environment so far was so small it didn't pose any health threats.

A complete meltdown - the collapse of a power plant's systems and its ability to keep temperatures under control - could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.

Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. The severity of their exposure, or if it had reached dangerous levels, was not clear. They were being taken to hospitals.

Edano told reporters that a partial meltdown in Unit 3 of the Fukushima Dai-ichi power plant was "highly possible."

Asked whether a partial meltdown had occurred, Edano said that "because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption" that it had.

The government doubled the number of troops pressed into rescue and recovery operations to about 100,000 from 51,000, as powerful aftershocks continued to rock the country. Hundreds have hit since the initial temblor.

Unit 3 at the Fukushima plant is one of the three reactors that had automatically shut down and lost cooling functions necessary to keep fuel rods working properly due to power outage from the quake. The facility's Unit 1 is also in trouble, but Unit 2 has been less affected.

On Saturday, an explosion destroyed the walls of Unit 1 as operators desperately tried to prevent it from overheating and melting down.

Without power, and with its pipes and pumps destroyed, authorities resorted to drawing seawater mixed with boron in an attempt to cool the unit's overheated uranium fuel rods. Boron disrupts nuclear chain reactions.

The move likely renders the 40-year-old reactor unusable, said a foreign ministry official briefing reporters. Officials said the seawater will remain inside the unit, possibly for several months.

Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, told reporters that the seawater was a desperate measure.

"It's a Hail Mary pass," he said.

He said that the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution. He said the dousing would need to continue nonstop for days.