U.S. advised citizens to consider leaving Japan and to keep 50 miles (80 kilometers) away from the Dai-Ichi plant, about 135 miles north of Tokyo, while the U.K. said people should consider leaving the capital altogether

Small amounts of radioactive isotopes from the crippled Japanese nuclear power plant are being blown toward North America high in the atmosphere over the Pacific Ocean and will reach California as soon as Friday, according to experts.

A network of sensors in the U.S. and around the world is watching for the first signs of that fallout, though experts said they were confident that the amount of radiation would be well within safe limits.

Operated by the Environmental Protection Agency, the U.S. network known as Radnet is a system of 100 radiation monitors that work 24 hours a day, spread across the country in places such as Anaheim, Bakersfield and Eureka. In addition, a network of 63 sensors is operated by the Comprehensive Test Ban Treaty Organization, an international agency allied with the United Nations.Foreign governments heightened safety warnings to nationals in Japan as concerns mounted that authorities there are losing the battle to contain leaks from the quake-stricken nuclear plant in Fukushima.

The U.S. advised citizens to consider leaving Japan and to keep 50 miles (80 kilometers) away from the Dai-Ichi plant, about 135 miles north of Tokyo, while the U.K. said people should consider leaving the capital altogether. Japan’s Chief Cabinet Secretary Yukio Edano urged people not to panic.

“It’s important that the whole country stick together and overcome this situation, and nothing is more important than showing that to the world market,” Edano said.

Japanese bond risk soared to a record as pessimism grew that the repercussions of the strongest earthquake in the nation’s history would be worse than the government has signaled. Buildings in Fukushima were rattled through the night by aftershocks, as a snowstorm and shortages of fuel, water and food assailed the city of 290,000 living in the shadow of the unraveling nuclear crisis.

“We are struggling desperately to recover,” Yoshihiro Murata, the governor of the neighboring region of Miyagi, where more than 10,000 are estimated to have died, wrote in an email. “I am afraid it’s going to take a few years to revive. The disaster was far bigger than we prepared for.”

The magnitude-9 earthquake that hit March 11 sparked a 7- meter tsunami that engulfed the northeast coast. There were 5,178 confirmed dead, with 8,913 missing as of 12 p.m. Tokyo time today, the National Police Agency said. Buildings were razed to the ground as the wave carried a wall of cars, uprooted trees and other debris through coastal towns.