Polio has been detected in New York City's sewage, suggesting the virus that causes the disease is likely circulating there, health authorities said, calling on residents to make sure they are vaccinated.
State health commissioner Mary Bassett called the finding "alarming, but not surprising."
Last month, a polio case was reported in Rockland County, about 50km northwest of Manhattan - the first one in the United States in nearly a decade.
Traces of the virus were then found in the wastewater of both Rockland and a neighbouring county.
"For every one case of paralytic polio identified, hundreds more may be undetected," Ms Bassett said in a statement. Those who contract the virus but do not develop symptoms can transmit the highly contagious illness to others.
Polio, which notably affects children, can cause permanent paralysis of the arms and legs, and even death.
"The risk to New Yorkers is real but the defence is so simple - get vaccinated against polio," New York City's health commissioner Ashwin Vasan said.
Only 86% of children aged six months to five years in the US's most populous city have received three doses of vaccine, officials said - meaning 14% are not fully protected.
A vaccination site for children - with the jab provided for free or at low cost - has been set up.
Experts from the US Centers for Disease Control and Prevention have been dispatched to New York state to help lead testing and vaccination efforts.
"The consequences of polio are devastating and irreversible, and these latest findings are a cause for concern," the CDC said.
Polio has been nearly eradicated worldwide but in June, British authorities said they had found traces of it in London sewage samples, and stepped up vaccination efforts.
