Powerful Hurricane Joaquin is headed towards Bermuda after hammering the Bahamas and leaving a cargo ship with 33 crew members missing in its wake.
Meanwhile, vast swaths of the US southeast and mid-Atlantic states were grappling with heavy rains and flooding from a separate weather system which has already caused at least two deaths, washed out roads and prompted evacuations and flash flood warnings.
US President Barack Obama declared a state of emergency in South Carolina on Saturday, making federal emergency funds available.
At 5pm (21:00 GMT), Joaquin, which strengthened significantly earlier on Saturday, had maximum sustained winds of 240km per hour, the US National Hurricane Center (NHC) said.
The storm, a potentially catastrophic Category 4 on a scale of 1 to 5, was about 805km southwest of Bermuda, the Miami-based centre said.
It swirled away from the Bahamas early on Saturday, after slamming parts of the archipelago for more than two days.
The storm was expected to pass west of Bermuda - well off the US coastline - on Sunday, before heading on a north-northeast track taking it further out to sea.
Any slight eastward deviation in the forecast track could put Joaquin dangerously close to Bermuda, however, the NHC warned.
Life ring found
The US Coast Guard said the only trace on Saturday of El Faro, which went missing off Crooked Island in the Bahamas on Thursday morning after it was overcome by heavy weather from Joaquin, was a life ring from the 224-metre cargo ship.
The vessel, with 28 US citizens and five Polish nationals aboard, was headed to San Juan, Puerto Rico, from Jacksonville, Florida when it reported losing propulsion and that it was listing and taking on water, the coastguard said.
"We are very surprised that we lost all communication with the ship," Mike Hanson, a spokesman for El Faro's owner, Tote Maritime Puerto Rico, told the Reuters news agency on Saturday.
"The ship was simply in the wrong place at the wrong time," he added, saying Joaquin was just a tropical storm when El Faro set out from Jacksonville but later intensified rapidly into a major hurricane.
A statement from the North Carolina governor's office said up to 500 residents of Brunswick County had been evacuated from their homes Friday night into early Saturday morning due to flooding from heavy rains and a levee failure in South Carolina.
"It's definitely a life-threatening situation," said National Weather Service meteorologist Steve Pfaff.
"There were people that were stuck in vehicles that were flooded and water in some of the homes was up over the electrical outlets," he said.
No deaths or serious injuries were reported in the Bahamas due to Joaquin, which destroyed houses, uprooted trees and unleashed heavy flooding on several smaller islands, but two deaths in the Carolinas on Thursday were linked to rain there.
It was not clear whether the deaths of four people in a small plane crash Friday near Lake Hartwell, South Carolina, was weather-related.
Before an earlier shift in Joaquin's trajectory, New York and New Jersey, where superstorm Sandy killed more than 120 people and caused $70bn of property damage in October 2012, faced potential threats from the storm.