The study said the worst sulphur pollutants in the Arctic, by Russian metals smelters and industries far to the south, had declined in recent years with lower emissions. Many lakes and soils blighted by acid rain and snow were recovering, it said.

But some other toxins in the almost uninhabited region, including nitrogen oxides that may be carried by winds from industries or forest fires to the south, seemed to be rising.

A brownish haze, which can cut visibility in the near pristine Arctic in spring, had started to increase in the late 1990s after clearing since the 1970s, according to measurements in Barrow, Alaska. Haze levels were still below the 1980s.

"The cause of this recent increase is not yet known," the report said.

Reiersen told Reuters one theory was that: "The haze might be linked to climate change -- with increased temperatures there are more forest fires. That means more soot in the atmosphere."

Warmer temperatures in recent decades mean the forest fire season in northern forests starts earlier and ends later. Most scientists say fossil fuels burnt in power plants, factories and cars release heat-trapping gases that are raising temperatures.

CHINA

Pollution from growing economies such as China may be adding to haze, whose particles can also fall as acid rain or snow. "The importance of Asian sources to acidification and Arctic haze pollution ... is not yet clear," the report said.

The report said there was no sign of significant health damage to people, even those living near Russian smelters. The current level of deposition of acids "does not appear to be a threat to terrestrial ecosystems in most of the Arctic."

On the Kola peninsula in northwest Russia, there were signs of recovery. Emissions by the Nikel smelter, for instance, have long blighted forests, killed fish and destroyed lichen that is the staple food for reindeer.

The AMAP report urged Arctic Council nations -- Russia, the United States, Canada and the five Nordic states -- to consider tighter limits on emissions of industrial pollutants.

It said that existing international rules under the UN's Gothenburg Protocol, which sets ceilings on industrial pollutants until 2010, were no longer enough to guarantee a longer-term decline in acidification, it said.

An opening of the Arctic region to oil firms, and to sea transport if the polar ice shrinks with higher temperatures, may also bring more pollution. Some US studies say the Arctic may contain a quarter of the world's undiscovered oil and gas.

Arctic haze was first noted in the 1950s by Canadian pilots puzzled by low visibility over the pristine ice.