Five people are confirmed to have contracted the Zika virus in Tonga, with 259 others also suspected of having caught it.
The virus, which causes flu-like symptoms including high fever and a rash, has been linked to babies being born with birth defects including microcephaly, or an abnormally small head.
The Center for Disease Control and Prevention in the US added Tonga to its travel advisory list today, joining American Samoa and Samoa.
Many countries in South America have now been confirmed as having mosquitoes actively transmitting the virus, while a case has also today been confirmed in Spain.
Several other Pacific islands have seen Zika outbreaks in recent years, most notably French Polynesia and Vanuatu.
The World Health Organisation advises people in countries with a confirmed Zika presence to use mosquito nets, protective clothing and insect repellent.