The periodic phenomenon known as El Nino has developed in the Pacific Ocean threatening extreme weather in many parts of the world, US scientists say.

El Ninos begin with a warming of waters in the eastern Pacific, and there has been a steep rise in water temperature in recent weeks, they say.

This El Nino is likely to strengthen towards the end of the year and early into 2007, the researchers add.

However it is not expected to reach the strength of the 1997 phenomenon.

In that year El Nino brought drought to parts of Asia and Australia, and heavy rains and floods to Latin America.

Scientists from the US National Oceanic and Atmospheric Administration (Noaa) say there has been drier-than-average weather in Indonesia, Malaysia and the Philippines - countries which are often the first to show the effects of a new El Nino.

But the early indications are that weather changes will be milder than in some previous events.

"What happens is that the cool current over the eastern Pacific, which brings cold waters from the Antarctic, up the South American coast towards the equator - the Humboldt currrent - weakens, and this allows El Nino to develop off South America, and the temperatures rise quite considerably," commented Dr Harvey Sterne from the Australian Bureau of Meteorology.

"In some years we've had four or five degrees Celsius above normal; now, this year, they're about one or two degrees above normal, so it's nothing like we had in the early 1980s."

Noaa says this latest phenomenon may explain why this year's Atlantic hurricane season has so far been weaker than expected - winds associated with El Nino events disrupt and weaken storm formation.

The researchers are also predicting a milder-than-average winter for much of North America, and wetter weather for the US Gulf Coast and Florida.