Hurricane Isaias slammed into North Carolina Monday as a category one storm, with winds of up to 85 miles (140 kilometers) an hour, the US National Hurricane Center said.
The storm came ashore in the southern part of the state near Ocean Isle Beach at 11:10 pm (0310 GMT Tuesday).
Meteorologists warned of heavy rain and potentially life-threatening storm surges.
The storm is expected to track up the East Coast, with heavy rainfall expected to cause flash flooding through the mid-Atlantic states and bringing storm-force winds to Washington, Philadelphia and New York on Tuesday.
Residents in flood-prone areas "should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions," the National Hurricane Center said.
In the hurricane warning zone, "preparations to protect life and property should be rushed to completion," it added.
President Donald Trump announced he had issued emergency declarations for Florida and both Carolinas, to free up federal funds.
"Everyone needs to remain vigilant," he warned during a news conference.
"I encourage everyone to follow the guidelines of the state and local authorities," he said, adding the guidelines have been "very accurate so far."
"So stay safe," he added.
'Pack your masks'
The Carolinas have seen a recent surge in coronavirus cases as the US struggles to curb the spread of the potentially fatal illness.
"We in North Carolina mostly know what to do," the state's governor, Roy Cooper, said Sunday.
"You pack your emergency kit, follow local evacuation orders, stay in a safe place and never drive through flooded roadways.
"But this time, pack your masks and hand sanitizers in your kit and remember to social distance."
Up to 150 National Guard soldiers have been activated to help prepare for the storm and assist in the aftermath if necessary, Cooper said.
Isaias was earlier downgraded from a Category 1 hurricane after brushing past Florida, leaving it relatively unscathed.
Some coronavirus testing centers -- many housed in tents -- were closed last week in the Sunshine State as the storm approached.
They have now begun to reopen gradually, according to local media.
Florida has emerged as an epicenter of the US virus crisis, which has already claimed nearly 155,000 lives nationwide.
On Sunday, the state reported 77 COVID-19 deaths, down from a record 179 the day before, bringing its death total to 6,920.
The storm earlier dumped torrential rain on the Bahamas, felling trees and flooding streets, before emergency management officials on Sunday gave the "all clear" for the country.
At least one person died in Puerto Rico and the storm also lashed Haiti and the Dominican Republic.