Surveillance of mosquito vectors in recent weeks have showed increasing incidence of the disease, health officials say.

Between June 10 and 16 alone, the city admitted 77 dengue patients, nine more than the previous week. But no deaths have been reported this year.

On Friday, the Hanoi Center for Disease Control predicted that in the coming days, temperatures ranging between 28 and 36 degrees Celcius (82.4 - 96.8 degrees Fahrenheit) and scattered rain will form favorable conditions for mosquitoes to spread the fever.

Hanoi has been going through one of its hottest years. The city has been hit by a heat wave on recent days, with temperatures on Saturday peaking at 40 degrees Celsius. In mid-April, its temperatures peaked at 38.9 degrees (102 degrees Fahrenheit) - the highest in 100 years, compared to the average April temperature of about 24 degrees Celsius.

In the first nine months of last year, Vietnam recorded more than 64,000 cases of dengue fever, including 11 deaths. Corresponding figures for 2017 was 183,287, including 30 deaths.

The country has been trying to find an effective way to tackle dengue fever, one of the biggest killers out of its 28 common infectious diseases.

In May, the U.S. approved the first vaccine against dengue, Dengvaxia, manufactured by Sanofi Pasteur, but it only be used in individuals aged 9 to 16 living in parts of the United States where the dengue virus is endemic.