All 20 cases are being treated at the city’s National Hospital of Tropical Diseases, Xinhua news agency reported.
Two cases are progressing seriously with multiple infections, the Department said.
Other patients have positive health progress, Vietnam Television (VTV) reported.
According to the VTV, the city’s healthcare sector has not detected foreign-related factors linked to current monkeypox cases.
Local healthcare authorities are stepping up measures to detect community infection, focusing on easily-infected groups at hospitals and preventing the virus entering from outside through border gates.