* Reykjavik first detection of Japan radioactivity in Europe
* Radioactive particles headed eastwards via U.S. from Japan
* Radiation far below harmful levels - experts
* Seen spreading over northern half of globe
(Recasts with Icelandic confirmation)
VIENNA/OSLO, March 22 (Reuters) - Minuscule numbers of radioactive particles believed to have come from Japan's crippled Fukushima nuclear power plant have been detected as far away as Iceland, officials said on Tuesday.
Traces of iodine-131, below levels of concern for human health, were discovered in an air filter used at a radiation monitoring centre in Reykjavik at the weekend, the Icelandic Radiation Safety Authority (IRSA) said.
"We believe it comes from Japan," Sigurdur Emil Palsson, head of emergency preparedness at IRSA, told Reuters, confirming an earlier account by diplomatic sources in Vienna and marking the first detection in a European nation.
Iodine-131 had also been recorded in places including Newfoundland in Canada and the western United States, consistent with a spreading plume, he said. Iodine-131, linked to cancer if found in high doses, contaminates products such as milk and vegetables.
The Fukushima plant leaked radiation after it was crippled in the March 11 tsunami and earthquake.
"It's only a matter of days before it disperses in the entire northern hemisphere," said Andreas Stohl, a senior scientist at the Norwegian Institute for Air Research.
"Over Europe there would be no concern about human health."
Palsson added: "From a health aspect the focus is on Japan."
Earlier, the diplomatic sources said tiny traces were picked up in Iceland by a network of international monitoring stations as they spread eastwards with winds from Japan across the Pacific, North America and the Atlantic.
"They measure extremely small amounts," one Vienna-based diplomat said. "It has nothing to do with any health risks."
The Comprehensive Test Ban Treaty Organisation (CTBTO), a Vienna-based U.N. body for monitoring possible breaches of the atom bomb test ban, has 63 stations worldwide for observing such particles, including one in Reykjavik, the Icelandic capital.
The CTBTO continuously provides data to its member states, but does not make the details public.
Another source said about 15 CTBTO stations had so far detected particles believed to originate from Fukushima.
"Reykjavik is the first in Europe," the source added.
NOWHERE NEAR CHERNOBYL LEVELS
The U.S. Energy Department and the Environmental Protection Agency late last week confirmed "minuscule" amounts of radiation that appeared to have come from Japan's damaged reactors were detected in California, where the CTBTO also has a station.
They said the radiation amounted to one-millionth of the dose rate that a person normally receives from natural sources such as rocks, bricks and the sun.
France's nuclear safety authority ASN said tiny radiation concentrations, perhaps 1,000 or 10,000 times less than from the 1986 Chernobyl disaster, were set to reach the country on Wednesday.
While only minor traces of radiation have been detected in countries outside Japan, the U.N. nuclear watchdog said on Monday "high levels of contamination" have been measured around the Fukushima plant itself.
At the site on Tuesday, smoke and steam rose from two of the most threatening reactors at the quake-crippled nuclear plant, suggesting the battle to avert a disastrous meltdown and stop the spread of radiation was far from won. (Editing by Mark Trevelyan)