Hanoi - Vietnam's largest city has experienced 38 cases of the Zika virus, state media reported Tuesday.
Three new cases publicised by health officials in Ho Chi Minh City follow six cases reported on November 11, the Tuoi Tre newspaper reported.
Out of the city's 24 districts, patients have been detected in 13.
Nguyen Huu Hung, deputy director of the Ho Chi Minh City Department of Health, attributed the outbreak to poor environmental hygiene in the affected districts.
Zika is a virus causing symptoms similar to the flu. While most Zika patients recover quickly, infection during pregnancy is believed to cause microcephaly in children.
While no cases of microcephaly have been detected in Ho Chi Minh City, the government reported in late October that a 4-month-old girl in Dak Lak province was likely Vietnam's first case of Zika-linked microcephaly.
The US Centre for Disease Control recommends pregnant women consider postponing travel to Vietnam and other Asian countries.