The first official estimate since the March 11 disaster covers damage to roads, homes, factories and infrastructure, and dwarfs losses from both the 1995 Kobe quake and Hurricane Katrina that swept through New Orleans in 2005, making it the world's costliest natural disaster.

As concern grew over the risk to food safety of radiation from the crippled Fukushima power plant, 250 km (150 miles) north of the Japanese capital, the United States became the first nation to block some food imports from the disaster zone.

The plant, battered by a 9.0 magnitude earthquake and tsunami that has left 23,000 people dead or missing, has still not been brought under control, and workers were forced away from the complex when black smoke began rising from one of its six reactors.

Tokyo authorities said that water at a purification plant for the capital of 13 million people had 210 becquerels of radioactive iodine -- more than twice the safety level for infants.

Tokyo Governor Shintaro Ishihara, however, said that level posed no immediate risk and water could still be used. "But, for infants under age one, I would like them to refrain from using tap water to dilute baby formula," he added.

The US Food and Drug Administration said it was stopping imports of milk, vegetable and fruit from four prefectures in the vicinity of the stricken nuclear plant.

South Korea may be next to ban Japanese food after the world's worst atomic crisis since Chernobyl in 1986. France this week asked the European Commission to look into harmonizing controls on radioactivity in imports from Japan.

Food made up just 0.6 percent of Japan's total exports last year.

Authorities have said above-safety radiation levels had been discovered in 11 types of vegetables from the area, in addition to milk and water.

Chief cabinet secretary Yukio Edano, who has been the government's public face during the disaster, said an exclusion zone around the plant did not need to be expanded, and he urged Tokyo residents not to hoard bottled water.

Earlier, he said there was no major danger to humans and he urged the world not to overreact: "We will explain to countries the facts and we hope they will take logical measures based on them."

Japan has already halted shipment of some food from the area and told people there to stop eating leafy vegetables. Asian neighbors are inspecting imports for contamination, and Taiwan advised boats to stop fishing in Japanese waters.

At the Fukushima plant, engineers battling to cool reactors to contain further contamination were evacuated when black smoke rose from the No.3 reactor, the plant's operator, Tokyo Electric Power Co, said. It did not know what caused the smoke.

Other Asian neighbors are inspecting imports for contamination and Taiwan advised boats to stop fishing in Japanese waters.

At the Fukushima plant, engineers battling to cool reactors to contain further contamination were evacuated when black smoke rose from the No.3 reactor, the plant's operator, Tokyo Electric Power Co. (TEPCO), said. It did not know what caused the smoke.

A government official later told reporters the smoke was not a serious issue.

The Asian nation's worst crisis since World War Two has sent shock waves through global financial markets.

The damage estimate of $300 billion could go higher as it does not include losses in economic activity from planned power outages or the broader impact of the nuclear crisis. The 1995 Kobe quake cost $100 billion while Hurricane Katrina caused $81 billion in damage.

More than a quarter of a million people are living in shelters, while rescuers and sniffer dogs comb debris and mud looking for corpses and personal mementos.