The El Nino weather phenomenon, one of the worst in 50 years, has caused intense drought in southern Africa that will have a "devastating" impact on the region's food security, the UN food agency warned on Friday.

Across large parts of Zimbabwe, Malawi, Zambia, South Africa, Mozambique, Botswana, and Madagascar, the rainfall season has so far been the driest in the last 35 years, the UN Food and Agriculture Organization (FAO) said in a report.

The region's "intense drought... has expanded and strengthened since the earliest stages of the 2015-2016 agricultural season, driven by one of the strongest El Nino events of the last 50 years".

El Nino is the name given to a weather pattern associated with a sustained period of warming in the central and eastern tropical Pacific which can spark deadly and costly climate extremes in parts of the world.

The phenomenon is "set to have a devastating impact on harvests and food security" in southern Africa, the FAO report said.

It said much of the region has already experienced delays in planting and very poor conditions for early crop development and pasture re-growth.

Forecasts from a variety of sources "are unanimous in predicting a continuation of below-average rainfall and above-average temperatures across most of the region for the remainder of the growing season.

"The combination of a poor 2014-2015 season, an extremely dry early season (October to December) and forecasts for continuing hot and drier-than-average conditions through mid-2016, suggest a scenario of extensive, regional-scale crop failure."

It said South Africa, where drought emergencies have been declared in most provinces, has already issued a preliminary forecast of maize production for the coming harvest of 7.4 million tonnes, down 25 percent from the already poor production levels of last season.

Zimbabwe and Lesotho have also declared drought emergencies, and authorities in Botswana, Swaziland, South Africa and Namibia are limiting water usage because of low water levels.

"While it is too early to provide detailed estimates of the population likely to be food-insecure in 2016-2017, it is expected that the population in need of emergency food assistance and livelihood recovery support will increase significantly", the report said.