The seasonal hole in the ozone layer over Antarctica has widened to near-record size, the United Nations said Thursday, insisting though that efforts to save the earth's protective shield were still working.

The World Meteorological Organization said colder-than-usual high altitude temperatures over the Antarctic combined with ozone-eating gases lingering in the atmosphere had stretched the ozone hole to an average of 26.9 million square kilometres (10.4 million square miles) over a 30 day period -- covering an area larger than all of North America.

"This is the third largest observed after the record-breaking ozone holes in 2000 and 2006," the UN's climate and weather agency said in a statement.

The hole peaked on October 2, when it measured 28.2 million square kilometres, according to the measurements conducted by the US space agency NASA.

"This shows that the ozone hole problem is still with us and we need to remain vigilant," WMO scientist Geir Braathen said in the statement.

"But there is no reason for undue alarm."

The size of the hole in the ozone layer above the Antarctic fluctuates, usually peaking during the polar spring in September and October, when extremely cold temperatures mix with the returning sunlight to release chlorine radicals that destroy ozone.

WMO said that during colder years, the hole was larger, but stressed that "this does not reverse the projected long-term recovery in the coming decades."

The ozone layer -- which helps protect the Earth from potentially dangerous ultraviolet rays that can cause skin -- began developing holes on an annual basis starting in the 1980s due to widespread use of chlorofluorocarbons, or CFCs.

Once commonly used in refrigerators and aerosol cans, CFCs are now almost non-existent thanks to an international treaty signed in Montreal in 1987, amid global concern over widening holes in the ozone layer.

Last year, the UN said the ozone layer was "well on track" for recovery by mid-century, although fixing it over Antarctica would take longer.

"The Montreal Protocol is in place and is working well," Braathen said.

He warned though that "we may continue to see large Antarctic ozone holes until about 2025 because of weather conditions in the stratosphere and because ozone depleting chemicals linger in the atmosphere for several decades after they have been phased out.