MINGORA: An outbreak of dengue fever in northwest Pakistan's Swat valley has killed 23 people in the past month, but health officials said Thursday it was likely to subside as the weather cools in the coming weeks.

The authorities in Swat declared a health emergency a week ago over the outbreak of the virus, spread by mosquitoes which breed in stagnant water, which has now infected 6,500 people since late August.

But a local official with the World Health Organization (WHO) said that with the onset of cooler autumn weather in the mountainous region, infections should decrease.

“The number of dengue cases is decreasing and the countdown has been started,” Doctor Qutbuddin Kakar, the WHO's focal person for dengue, told AFP.

“We are expecting more visible decline in the coming days with the change in the temperature.”Health officials said the mosquitoes carrying dengue could not survive in temperatures below 16 Celsius.

It is thought the Swat outbreak was caused by infected mosquito larvae brought from the eastern city of Lahore in water in a consignment of old tyres.

An outbreak in Lahore, Pakistan's second largest city, in September 2011 killed 362 people and infected more than 21,000.