El Nino, a warming of water surface temperatures in the eastern and central Pacific Ocean, has officially returned after three years of the La Nina climate pattern.
This is likely to yield extreme weather later this year, from tropical cyclones spinning toward vulnerable Pacific islands to heavy rainfall in South America to drought in Australia and in some parts of Asia.
"WHO is preparing for the very high probability that 2023 and 2024 will be marked by an El Nino event, which could increase transmission of dengue and other so-called arboviruses, such as Zika and chikungunya," the agency's director-general Tedros Adhanom Ghebreyesus said on Wednesday.
The WHO chief warned climate change was fuelling the breeding of mosquitoes, and incidence of dengue has already risen sharply in recent decades, particularly in the Americas.
Peru has declared a state of emergency in most regions this year and its health minister Rosa Gutierrez last week resigned amid a surge in cases of dengue.
The disease is transmitted through the bites of aedes aegypti mosquitoes, with symptoms that include fever, eye, head, muscle and joint pain, nausea, vomiting and fatigue.
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