"It could be compared to a building in a movie: it looks OK, you see a building, but in fact it's a set, and behind the facade there is nothing," said Walter Meier, of the National Snow and Ice Data Center at the University of Colorado in Boulder.

"In February, there was a slight recovery, surface area of ice was slightly higher than in the recent years, because of a colder than average winter, but ice area is only one parameter," he told a joint press conference with National Aeronautics and Space Administration (NASA) researchers.

"It is seasonal ice, thin, vulnerable which could melt very easily," Meier added.

"The area of perennial ice decreased," he said in his report of measurements of the polar ice caps taken by NASA's ICESat satellite.

The polar ice cap consists of seasonal ice that expands and shrinks with winter and summer, and perennial ice that never melts.

Meier said perennial ice was "an indicator of the long-term health of the ice," adding that "it's not looking very good."

He said perennial ice covered 50-60 percent of the Arctic Ocean some 20 years ago, while in 2008 it had shrunk to 30 percent.

Global warming caused by man-made greenhouse gas emissions is largely blamed for retreating glaciers and thinning polar ice cover around the world.

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