Hurricane Joaquin, the third of the Atlantic season, has strengthened into a category 3 storm as it nears the Bahamas, the US National Hurricane Center (NHC) says.

The eye of the storm is expected to pass over eastern islands of the Bahamas overnight.

A hurricane warning is in effect for much of the country, that could see winds of up to 185km/h (115mph).

The NHC says Joaquin could affect the US East Coast by Sunday.

It warned that Joaquin "could become a major hurricane" by Friday. It picked up considerable strength on Wednesday, after being upgraded from a storm to a category 3 hurricane, on a scale of five, in only a few hours.

Geoffrey Greene, a senior forecaster with the Bahamas Meteorology Department, said he was "very concerned" about some of the smaller islands in Joaquin's path, such as San Salvador, Rum Cay and Cat Island, which all have small populations.

Officials in New York, New Jersey and Connecticut, states badly affected by Superstorm Sandy in 2012, warned residents should begin making preparations.

Virginia's Governor, Terry McAuliffe, has declared a state of emergency in response to heavy flooding earlier in the week, and because of what he called "a serious threat to life and property" from Joaquin.

A White House spokesman said the Federal Emergency Management Agency (Fema) was following the progress of Joaquin and preparing in case it made landfall in the US.