The United Nations treaty to protect the ozone layer signed nearly 25 years ago prevented an environmental disaster, a chief UN scientist said Friday, cautioning though that the Earth's radiation shield is still under threat.

"The Montreal Protocol has prevented a major environmental disaster," Gael Braathen, the World Meteorological Organization's senior scientific officer for atmospheric environment research, told reporters in Geneva.

The treaty was signed on September 16, 1987, amid growing concern over swelling holes in the ozone layer, which filters out ultraviolet rays that damage vegetation and can cause skin cancer and cataracts.

It banned ozone-depleting substances such as chlorofluorocarbon gases (CFCs), once present in things like refrigerators and spray cans.

Since then, ozone depletion has levelled off, Braathen said, adding though that it would still take a very long time for the ozone layer to recover.

"As we speak, ozone depletion is going on," he said, adding that "we haven't really seen any kind of unequivocal recovery yet".

In the Arctic, record ozone damage was reported in the stratosphere in 2011, but levels normalised in 2012, he said.

"Ozone-depleting gases have a long lifetime in the atmosphere so it will take some decades before the ozone is back to where it was in the past," he added.

According to the WMO's figures, the amount of ozone-depleting gases in the Antarctic reached a peak in the year 2000. The amount is now decreasing at a rate of about 1.0 percent a year.

The ozone layer outside the polar regions is projected to recover to its pre-1980 levels before the middle of this century, the WMO said.

In contrast, the ozone layer over the Antarctic is expected to recover much later.