The virus has been linked to a surge in births of babies with abnormally small heads, and pregnant women have been advised not to travel to Latin American and Caribbean countries because of the virus.
"An infected person must have come, who infected a population of mosquitos, which in turn infected a local population, permitting the spread of the disease," Puerto Rico's Health Secretary Ana Rius said.
She reported that 18 new cases of the Zika virus have been detected in the US possession, raising the total to 19 since the Caribbean island's first case was confirmed last month.
So far the virus has not been detected in the continental United States. However, a number of residents have contracted it while traveling and officials are concerned that it could spread if they are bitten by mosquitoes after returning home.
Arkansas became the latest state to raise concerns yesterday after tests showed a resident had contracted a mild case of Zika while abroad. Florida, Hawaii and New York have also reported cases in recent weeks of residents who contracted Zika after they traveled to areas impacted by the virus.
Puerto Rico's cases of Zika, a virus carried by the Aedes aegypti mosquito which also transmits dengue and chikungunya, were concentrated in the island's southeast, Rius said.
In Brazil, Zika virus infections in pregnant women have been linked to 3,893 births of babies with abnormally small heads, a condition known as microcephaly. "We appeal especially to pregnant women, that if they have symptoms of fever, to go immediately to their doctor," Rius said.
The World Health Organisation has warned that the virus could spread rapidly throughout the western hemisphere, except in Chile and Canada, which have no Aedes aegypti mosquitos. US health authorities have warned pregnant women not to travel to 22 places in Latin America and the Caribbean to avoid exposure to the Zika virus.