President Barack Obama said Sunday that Irene which had been downgraded from a category one hurricane to tropical storm is still "dangerous" as the public is at risk of flooding and power outages.

In his White House address, Obama said the federal government remains concerned about localized flooding, Xinhua reported.

"This is not over," he said, adding that the impact of Irene will be felt for some time and some areas would take some time to recover from the storm's onslaught.

The president pledged that federal agencies will "do everything in their power" to help the affected communities.

CNN reports: Even as strong winds and heavy rains from Irene continued to whip parts of northern New England, millions of residents all along the East Coast were in recovery mode Sunday afternoon, some of them still battling floodwater after the drenching storm.

Irene, which was a hurricane for days before weakening to tropical storm status earlier Sunday, caused at least 15 deaths across six states and knocked out power to more than 4 million people. Yet some of the biggest headaches related to flooding -- as tidal storm surges and overflowing rivers left homes in North Carolina and points northward awash, with even a Vermont official reporting "terrible" conditions.

"I want people to understand that this is not over," President Barack Obama said at 5 p.m. Sunday from Washington. "The impacts of this storm will be felt for some time, and the recovery effort will last for weeks or longer."

New Jersey was particularly hard-hit, leaving residents including Guy Pascarello -- whose family's Secaucus home of 40 years was declared uninhabitable after it became inundated by three-foot-high waters -- trying to figure out what to do next.

Some 3,000 workers from New Jersey's transportation department and sister agencies worked Sunday to clear debris and address flooding in more than 300 locations around the state, the department said.

Even locations well inland, like Princeton Junction about halfway between New York City and Princeton, had waters as high as 12 feet that covered roads and bridges, resident Edward Picco said. And streets in downtown Millburn saw major flooding when the Rahway River overflowed early Sunday morning, said Lt. Peter Eakley, the town's deputy emergency management coordinator.

In New York City's lower Manhattan, the Hudson River overflowed. It later receded -- but not before sending massive amounts of water spilling over jogging paths and pouring into at least one nearby apartment building. Water also lapped over the banks of the city's East River early Sunday, but later receded.

Mayor Michael Bloomberg reported "serious flooding across the five boroughs," including in Orchard Beach and riverfront parking lots in the Bronx. He noted that there had been "substantial erosion" on beaches in Staten Island and Rockaway, and that strong winds uprooted about 650 trees.

Firefighters helped evacuate dozens of people from flooded homes in areas of Staten Island where neck-deep water was reported, the New York City Fire Department said.

And Metropolitan Transit Authority chief Jay Walder said lines on the Metro-North system had flooded, eroding tracks and causing significant damage.

Outside Philadelphia, meanwhile, waters had already climbed to street-sign levels in Darby, Philadelphia Mayor Michael Nutter said, with the water sending "couches, furniture, all kinds of stuff floating down the street." Two buildings collapsed in Philadelphia, Nutter told reporters, but no one was injured.

By 5 p.m., Tropical Storm Irene had maximum sustained winds of 50 mph, according to the center. The storm was moving over northern New England, with its center about 65 miles south of Rutland, Vermont. The hurricane center warned about persisting "elavated water levels ... along the coast from Connecticut through Maine" that will be accompanied by "large, destructive and life-threatening waves."

The U.S. government said wind damage alone is expected to top $1 billion.