The United Nations' weather and climate agency said El Nino conditions were likely as early as the May to July three-month window.

The World Meteorological Organization (WMO) meanwhile said early signs indicated a strong event.

El Nino is a naturally-occurring climate phenomenon that warms surface temperatures in the central and eastern equatorial Pacific Ocean. It brings changes in winds, pressure and rainfall patterns.

Conditions oscillate between El Nino and its opposite La Nina, with neutral conditions in between.

The last El Nino contributed to making 2023 the second-hottest year on record and 2024 the all-time high.

"After a period of neutral conditions at the start of the year... there is high confidence in the onset of El Nino, followed by further intensification," said Wilfran Moufouma Okia, the WMO's climate prediction chief.

"Models indicate that this may be a strong event -- but... forecast confidence generally improves after April."

El Nino typically occurs every two to seven years and lasts around nine to 12 months.

The WMO said its latest monthly Global Seasonal Climate Update showed a clear shift in conditions in the equatorial Pacific.

Sea surface temperatures are rising rapidly, "pointing to a likely return of El Nino conditions as early as May-July", it said.

Forecasts indicate a "nearly global dominance of above-normal land surface temperatures" in the next three months.

"There is no evidence that climate change increases the frequency or intensity of El Nino events," the Geneva-based WMO said.

"But it can amplify associated impacts because a warmer ocean and atmosphere increases the availability of energy and moisture for extreme weather events such as heatwaves and heavy rainfall."