Irene strengthened into the first hurricane of 2011.

Irene strengthened into the first hurricane of 2011. (CNN)

MIAMI, FL -- Hurricane Irene intensified into a Category 3 storm Wednesday as it battered the Bahamas and threatened to lash much of the United States' East Coast in coming days.

Irene could reach Category 4 strength by Thursday, the National Hurricane Center said.

The storm had maximum sustained winds of 120 mph as of 2 p.m. ET Wednesday, and is expected to strengthen "in the next day or so," the National Hurricane Center said .

"This is a huge storm," said CNN Meteorologist Jacqui Jeras. "The cloud field is more than 800 miles across. The tropical storm-force winds extend out 200 miles from the center."

The storm could threaten large sections of the Eastern Seaboard, from the Carolinas into the Northeast.

The strengthening, which officially made Irene a major hurricane, came as the storm continued to pound the southeastern Bahamas and the Turks and Caicos Islands.

Irene's eye was over Crooked Island in the southeastern Bahamas, the Miami-based Hurricane Center said in its 2 p.m. advisory. It was moving northwest near 12 mph.

"On the forecast track, the core of Irene will move across the southeastern and central Bahamas through tonight and over the northwestern Bahamas on Thursday," the advisory said.

In Nassau, in the northwestern Bahamas, many people had evacuated by Wednesday morning. Shopkeepers were boarding up their storefronts, and residents prepared to hunker down for the storm's arrival.

"We've been through quite a few hurricanes so were pretty much prepared," said Brian Nottage, who owns a shop and ice cream parlor in downtown Nassau.

Earlier, the storm unleashed heavy rain across northern Haiti, causing flooding, the United Nations said Wednesday.

There were no reports of major losses or damage, U.N. officials said.

The storm will curve northward as it nears the United States, with most of the latest projections suggest the storm will avoid landfall until it is north of North Carolina.

"However, this is a very dangerous storm and much of the East Coast, including North Carolina, should be prepared for a landfall," Jeras said.

North Carolina officials expect the storm to weaken to a Category 2 before it moves near Cape Hatteras on Saturday afternoon.

Coastal areas could see approximately 6 inches of rain and residents should expect tropical storm force winds.

Some computer models suggest New York or New Jersey could be hit.

"Everywhere from North Carolina to Massachusetts remains in the cone of uncertainty," Jeras said. "Worst-case scenario, we could be looking at two landfalls, or we could be lucky and get a brush instead of a direct hit. ... Even if Irene doesn't make landfall in the United States, it may very well bring flooding rains, damaging winds and power outages to the Northeast. Planning is critical and everyone needs to be ready with a disaster plan and a safety kit."

The storm approaches as Americans increasingly are relying on social media to prepare for and keep abreast of disasters. According to two American Red Cross surveys, the increased use of social media and mobile technology has caused response agencies "to engage with people in times of disaster and to include information from social networks in their response efforts."