Authorities in Swat Valley have declared a health emergency after nearly 5,000 cases of dengue fever were reported in a month.

The mosquito-borne virus has killed 12 people in the valley since August 19 and the number of patients is increasing every day, doctors said.

Dengue is spread by mosquitoes that breed in stagnant water and usually bite people during the daytime. It causes high fever, severe joint and muscle pains and can be fatal.

"So far 12 people have died due this disease and 4,934 persons have been tested positive, we have declared a health emergency in the district," doctor Abdul Khaliq, district health officer in Swat told AFP by telephone.

Khaliq said the outbreak has been declared as an epidemic.

"It is becoming dangerous. Dengue larvae is everywhere, in and outside houses, in streets and all other places," Khaliq said.

Officials said they believed infected larvae had been 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 120 people and infected more than 1,200.

Hospitals in Mingora, the main town in Swat, are struggling to cope with the influx of patients, who complained of a lack of health facilities and basic medicines.

"We have not enough beds, no medicines and sometimes we go outside to have blood tests in private laboratories," Hamid Khan, a relative of one patient, told AFP.

"My brother had fever and we are worried because the blood test was positive."

There is no specific treatment for dengue but proper medical care for severe cases greatly reduces the risk of death, according to the World Health Organization.

Officials in Mingora's main Saidu Sharif hospital said that a special 100-bed ward has been set up to deal with dengue cases.