Singapore has confirmed two cases of locally transmitted infections of the Zika virus in a northeastern part of the city state, the National Environment Agency said on Wednesday, describing it as the first Zika cluster of 2017.

The viral disease carried by mosquitoes has spread to more than 60 countries and territories since an outbreak was identified in Brazil in 2015, raising alarm over its ability to cause microcephaly as well as Guillain-Barre syndrome.

Singapore had already reported six cases in the first 12 weeks of this year, the agency’s website shows. Last year, more than 400 people became infected with the virus, following discovery of the first case in August.

“Both cases are residents in the vicinity and from the same household,” the agency said in a statement.

“Residents and stakeholders are urged to maintain vigilance and continue to eliminate mosquito breeding habitats.”

There is no preventive treatment against Zika, but drug companies are rushing to develop a vaccine.

Although generally a mild disease, the virus is a particular risk to pregnant women as it can cause microcephaly – a severe birth defect in which babies are born with abnormally small heads and underdeveloped brains.