Radioactive isotopes are being blown toward North America "high in the atmosphere over the Pacific Ocean" and will reach California as soon as Friday, a report in the Los Angeles Times said.
"The material should begin showing up on the (US) West Coast as early as Friday, though it could take up to an additional week for the 5,000-mile trip from Japan to Southern California," the report said.
US network of sensors, known as the ''Radnet'' operated by the Environmental Protection Agency and comprising 100 radiation monitors across the country, are monitoring the level of radiation that is flowing from Japan towards the US west coast.
The EPA said it was monitoring the situation closely.
Its Radnet system had not yet detected radioactivity and it has added additional portable radiation monitors in Guam, Alaska and Hawaii.
US President Barack Obama also assured Americans that there was no danger of radiation for the country.
"I want to be very clear: We do not expect harmful levels of radiation to reach the United States, whether it''s the West Coast, Hawaii, Alaska, or US territories in the Pacific," Obama said in Washington today.
"Let me repeat that: We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska, or US territories in the Pacific. That is the judgment of our Nuclear Regulatory Commission and many other experts." The LA Times report said Nuclear Regulatory Commission, which oversees the US nuclear industry, "did not expect dangerous levels of radioactivity to hit the West Coast, Hawaii, Alaska or US territories in the Pacific." The radioactive levels that reach the US could increase over time since emissions from the damaged Fukushima Daiichi nuclear power plant have grown since the disaster broke out last week, the report added.
Experts have, however, said the particles would "wash out of the atmosphere" before they reach the US.
A specialist at nuclear watchdog group Union of Concerned Scientists Edwin Lyman said the US was not at any serious risk.
"We can never say never. My judgement is that there will probably be measurable radiation, but except for a few hot spots it is not something we should really worry about," the report quoted him as saying.