Radioactive contaminants spewed from the damaged Fukushima nuclear power plant in Japan have reached the open seas, according to a new report issued this week by U.S.-based researchers.

The Woods Hole Oceanographic Institution in Massachusetts reported of Tuesday that traces of the substance cesium-137 was detected in the Pacific Ocean and some 400 miles off the coast of the Japanese island that houses the power plants inundated by tsunami waves last year.

Gigantic waves swamped the northeast coasts of Japan when a massive quake struck the country March last year, devastating a sizeable portion of the nation and killing thousands on its wake.

Yet weeks after the initial shock, Japan grappled with threats of a nuclear fall out that had the world watching in fear when its damaged nuclear facilities teetered into uranium meltdown.

The second round of disaster was averted but experts conceded that radioactive contaminants were inevitably relased into atmosphere or via the sea water.

According to lead researcher Ken Buesseler, his team had detected considerable readings of cesium-137 on water samples, with levels that were 1000 times higher before the samples were collected June last year.

Buesseler, however, clarified that what has been gauged so far was from being alarming as "they were about one-tenth the levels generally considered harmful."

"We're not over the hump yet in terms of radioactive contamination of the ocean because of continued leakage from the plant," the scientist was quoted by the Associated Press (AP) as saying on Tuesday.

Buesseler's also noted that apart from cesium-137, other radioactive substances were also discharged from the Fukushima plant but scientists were focusing on the specific contaminant since it lingers in the environment much longer.

Cesium-137 has a projected half-life that lasts up to 30 years, Buesseler said.

He added that most likely, the contaminants reached the open seas through water discharges, eliminating suggestions that the area's surrounding atmosphere presently contains radioactive substances, or at least high levels of them.

The group's report, however, was welcomed by the International Atomic Energy Agency (IAEA), which called the latest findings as a bit of good news.

IAEA's Hartmut Nies told AP that the result was actually expected by the nuclear agency, adding that the amount of contamination detected will not pose any harm on human and marine lives.

"If it was not seawater, you could drink it without any problems," Nies said.

IAEA has been expecting for the contamination to decrease over time, Nies said, with the process largely expedited by the substances' dilution with the sea water.

"We still don't have a full picture but we can expect the situation will not become worse," the IAEA official told AP.