UPDATE 4-Low radioactivity seen heading towards N.America
* Particles not normal, but not dangerous-Swedish official
* U.S. nuclear body sees no "levels of harmful radiation"
* U.N. weather agency predicts northwesterly winter monsoon
(Adds U.S. comment)
By Fredrik Dahl
VIENNA, March 17 (Reuters) - Low concentrations of radioactive particles from Japan's disaster-hit nuclear power plant have been heading eastwards and are expected to reach North America in days, a Swedish official said on Thursday.
In Washington, the U.S. Nuclear Regulatory Commission (NRC) said radioactivity would disperse over the long distance and it did not expect any harmful amounts to reach the country.
"We expect the United States to avoid any levels of harmful radiation," NRC spokesman Joey Ledford told Reuters. "We do not anticipate any threat to American interests."
The Swedish official, research director Lars-Erik De Geer of the Swedish Defence Research Institute, was citing data from a network of international monitoring stations set up to detect signs of any nuclear weapons tests.
Also stressing the levels were not dangerous for people, he predicted particles would eventually also continue across the Atlantic and reach Europe.
"It is not something you see normally," he said by phone from Stockholm, adding the results he now had were based on observations from earlier in the week. But, "it is not high from any danger point of view."
De Geer said he was convinced they would eventually be detected over the whole northern hemisphere.
"It is only a question of very, very low activities so it is nothing for people to worry about," De Geer said.
"In the past when they had nuclear weapons tests in China ... then there were similar clouds all the time without anybody caring about it at all," he said.
De Geer said the main air movement in the northern half of the globe normally went from west to east, but suggested the direction occasionally changed and at times turned.
In Geneva, the U.N.'s World Meteorological Organisation (WMO) said on Thursday that a "northwesterly winter monsoon flow prevails over the eastern and northern part of Japan" and that this was expected to remain the case until around midnight GMT.
The New York Times earlier said a forecast of the possible movement of the radioactive plume showed it churning across the Pacific, and touching the Aleutian Islands on Thursday before hitting southern California late on Friday.
It said the projection was made by the Comprehensive Test Ban Treaty Organisation (CTBTO), a Vienna-based independent body for monitoring possible breaches of the test ban.
The CTBTO has more than 60 stations around the world which can pick up very low levels of radioactive particles such as caesium and iodine isotopes.
It continuously provides data to its member states, including Sweden, but does not make the details public.
De Geer said he believed the radioactive particles would "eventually also come here".
The New York Times said health and nuclear experts emphasized radiation would be diluted as it travelled and at worst would have extremely minor health consequences in the United States.
In a similar way, radiation from the Chernobyl disaster in 1986 spread around the globe and reached the west coast of the United States in 10 days, its levels measurable but minuscule. (Additional reporting by David Morgan in Washington; editing by Diana Abdallah)