The El Nino weather phenomenon could return within days but will be relatively weak compared with past episodes, the UN's weather agency said Tuesday.

The surface of the Pacific warmed in July and August, providing part of the mix necessary for El Nino to develop, the World Meteorological Organization (WMO) said.

There is a "moderately high likelihood for El Nino conditions to develop during September or October," the WMO said.

If it does develop, "it is most likely to be weak," the agency said, adding that it did not exclude "a moderate strength event" lasting into early 2013.

El Nino occurs every two to seven years, when the trade winds that circulate surface water in the tropical Pacific start to weaken.

A mass of warm water builds in the western Pacific and eventually rides over to the eastern side of the ocean.

The outcome is a major shift in rainfall, bringing floods and mudslides to usually arid countries in western South America and drought in the western Pacific, as well as a change in nutrient-rich ocean currents that lure fish.

It last occurred from June 2009 to May 2010, the Geneva-based agency said.

El Nino is followed by a return swing of the pendulum, a phenomenon called La Nina, which was declared over in April.

"No two El Ninos are the same," WMO spokeswoman Clare Nullis told AFP.

Even if conditions are ripe for another episode, "you can't say such and such a part of the world will be in drought... when a lot of other (weather) factors have an impact."