Charlie Harding / robertharding via Getty Images Health officials reported a dozen new Zika infections in Ho Chi Minh City as the virus spreads in Southeast Asia.

The number of confirmed Zika cases in Vietnam has more than doubled over the past three days to 23, with a dozen of the new infections recorded in the commercial hub of Ho Chi Minh City, the health ministry said on Wednesday.

The mosquito-borne virus has been spreading in Southeast Asia after outbreaks in the Americas. Thailand reported the region’s first confirmed case of microcephaly, a birth defect marked by small head size, linked to Zika in late September.

On Sunday, Vietnam’s health ministry reported its first microcephaly case that it said was likely linked to Zika.

The ministry said 14 more cases of Zika were reported since Sunday, most in the south of the country. Seventeen of Vietnam’s 23 cases have been in the south’s Ho Chi Minh City, the country’s biggest city.

Health officials were not immediately available for comment on the surge in cases.

Vietnam last month raised the threat level for Zika and stepped up monitoring of pregnant women.

Zika infections in pregnant women have been shown to cause microcephaly - a severe birth defect in which the head and brain are undersized - as well as other brain abnormalities.

The connection between Zika and microcephaly first came to light last year in Brazil, which has since confirmed more than 1,900 cases of microcephaly.

In adults, Zika infections have also been linked to a rare neurological syndrome known as Guillain-Barre, as well as other neurological disorders.

There is no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya and causes mild fever, rash and red eyes.

An estimated 80 percent of people infected have no symptoms, making it difficult for pregnant women to know whether they have been infected.