The UN atomic agency said Friday "very low levels" of radioactive iodine-131 had been detected in the air in the Czech Republic and in other countries, but presented no risk to human health.

The Czech nuclear safety office said the source of the contamination was "most probably" outside the Czech Republic, and that its information suggested the cause was not an accident at an atomic power plant.

Poland, Slovakia and Austria also said that they had detected abnormal but still very low levels, with Poland saying they had been "100 times higher" in March afer Japan's Fukushima nuclear accident.

According to a spokesman from Poland's National Atomic Energy Agency there were also "unconfirmed reports" about a possible incident at a nuclear power station in Pakistan.

The International Atomic Energy Agency said it had received information from Czech authorities "that very low levels of iodine-131 have been measured in the atmosphere over the Czech Republic in recent days.

"The IAEA has learned about similar measurements in other locations across Europe," it said, without saying which other countries were affected.

"The IAEA believes the current trace levels of iodine-131 that have been measured do not pose a public health risk and are not caused by the Fukushima Daiichi nuclear accident in Japan."

The Vienna-based agency said it was working with its counterparts to determine the cause and origin of the iodine-131, which has a half-life of around eight days, and that it would provide further information as it became available.

The Czech State Office for Nuclear Safety (SUJB) said "trace amounts" had been registered in the air in the past two weeks. The country has two nuclear power plants.

"The source of the contamination is not known at the moment," it said.

"We have not detected any increase in the concentration of other artificial radionuclides, which suggests the cause was not a nuclear power plant accident."

Dana Drabova, head of the Czech nuclear safety office, told the CTK news agency: "With probability bordering on certainty this is not from a Czech source, and definitely not from our nuclear plants."

In Poland, which has no nuclear power plants, a spokesman for the atomic energy agency told AFP: "We detected trace levels of radioactive iodine-131 over Poland during measurements taken October 17-24. It was a very low level.

"Readings were 100 times higher in late March in the wake of Japan's Fukushima nuclear accident," spokesman Stanislaw Latek said, adding that Ukraine had also detected "trace levels" between October 10-20.

He also said: "Unconfirmed reports suggest there may have been an incident at a nuclear power station in Pakistan but this requires further confirmation."

An IAEA spokesman said he had no information about any such incident, however.

On October 19, Pakistani authorities reported an emergency at the almost 40-year-old Karachi nuclear plant (KANUPP) when workers were forced to repair a leak.

In Slovakia, Vladimir Jurina from the public health authority said levels of iodine-131 were "just about measurable levels."

The values are basically the same as those in Austria, the Czech Republic and other countries. The source is a mystery to us. It's not from our nuclear plants though -- we have checked with them," he said.

In Austria, the environment ministry said "miniscule" levels of iodine-131 had been detected in the east and north of the country, posing no risk whatsoever to human health.

In Bulgaria, nuclear watchdog chief Sergey Tsochev said no traces of iodine-131 had been detected.

"Air samples remain normal," he said.