Four people suffering from the Zika virus in Florida are likely to be the first cases contracted within the United States.
The Florida department of health said "a high likelihood exists that four cases are the result of local transmission", centred on one small area just north of downtown Miami.
Until now cases outside Latin America and the Caribbean, where the virus is prevalent, have been spread by travel to that region or sexual contact.
But the authorities in Florida now consider it probable that the virus has been transmitted by mosquito bites in the area, meaning US mosquitoes may be carrying the virus.
More than 1650 cases of Zika have so far been detected in the United States.
Zika causes only a mild illness in most people but the virus has been linked to severe brain defects in newborns.
The US Centers for Disease Control and Prevention (CDC) said they expected more cases to emerge over the coming weeks. CDC chief Tom Frieden said infection occurred "several weeks ago" and that there were no plans to restrict travel to Florida.
"This is not just a Florida issue," Governor Rick Scott said. "It's a national issue - we just happen to be at the forefront."
To confirm whether Zika is being carried by mosquitoes locally, scientists are surveying houses and people within a 150m radius of the cases, the flying distance of the insect.
In February, the World Health Organisation declared the Zika virus a global public health emergency because of the risk to newborn children.
In severe cases, children can die and babies who survive can face intellectual disability and developmental delays.
Gov Scott asked all residents of affected areas to get rid of standing water, where mosquitoes thrive, and for residents to wear insect repellent.