GENEVA - Antarctic sea ice levels reached record lows last month, the World Meteorological Organization (WMO) said on Monday, a development climate change experts described as worrisome.

WMO said that Antarctic sea ice levels last month - the hottest June ever recorded -- were at their lowest since satellite observations began, at 17% below average.

"We're used to seeing these big reductions in sea ice in the Arctic, but not in the Antarctic. This is a massive decrease," Michael Sparrow, Chief of World Climate Research Programme, told reporters in Geneva.

Global sea surface temperatures were at record high for the time of the year in May and June, according to WMO, which warned that the warming of the world's oceans was spreading fast beyond their surface.

"It is not only the surface temperature, but the whole ocean is becoming warmer and absorbing energy that will remain there for hundreds of years," WMO said. "Alarm bells are ringing especially loudly because of the unprecedented sea surface temperatures in the North Atlantic."

The organization said that the El Nino weather pattern, which recently emerged, was expected to increase temperatures both on land and in the oceans, which could lead to more marine heatwaves and extreme temperatures.

(Reporting by Gabrielle Tétrault-Farber; editing by Christina Fincher)