Hurricane Irene may be history, but its effects will be felt for days to come, U.S. President Barack Obama said Sunday.

The former Category 1 hurricane, which has moved across the border into Canada, has now been reduced in strength to a post-tropical storm.

As Irene's strength waned late Sunday, forecasters said warnings for the U.S. east coast had all been called off.

In a brief speech at the White House, Obama said while the worst has passed, Irene will cause flooding in many areas for the next several days. He urged Americans to continue to listen to the guidance of their state and local officials in the coming days.

Obama praised those officials as well as first responders for their work preparing and responding to the storm, calling it an example of how good government can benefit the public.

Irene pushed northward along the U.S. coast on Sunday after causing flooding in New York City and widespread power outages in several states along the Atlantic seaboard. It lost its hurricane status as its centre crossed over Coney Island, N.Y., with winds of 104 km/h, but still delivered enough rain to cause flooding in Lower Manhattan.

As the storm moved north, commuters got some good news: New York City's subways will begin reopening Monday at 6:00 a.m. ET, though the service will be less frequent and trains more crowded, Gov. Andrew Cuomo and Metropolitan Transit Authority chairman Jay Walder said in a statement.

And in another travel-related development, officials said the three major New York-area airports will resume most flights Monday morning. Airlines had already resumed flights Sunday at many major Eastern airports. But a continuing closure of Kennedy, LaGuardia and Newark had threatened travel delays for much of the country.

A few hours before it made landfall in the New York City area, winds of up to 185 km/h whipped across the Eastern Seaboard, ripping power lines from poles and snapping trees in half. More than 4.5 million homes and businesses in several eastern states lost power.

Irene is being blamed for the deaths of at least 19 people from Florida to New Jersey, including two children, an 11-year-old boy in Virginia killed when a tree crashed through his roof and a North Carolina child who died in a crash at an intersection where traffic lights were out.

In Lower Manhattan, water from the East River began lapping over the seawall Sunday morning. Water poured into Battery Park at the southern tip of Manhattan. The Hudson River also overflowed its banks, flooding streets a block inland in the Meatpacking District, CNN reported.

New York power utility Consolidated Edison said flooding in Lower Manhattan due to storm surges appeared less severe than some forecasters had expected.

Over in the Long Island town of Long Beach, rain and high winds washed away protective sand berms lining the boardwalk, causing flooding of nearby streets. Flooding was also reported on the streets of Brooklyn near Sheepshead Bay.

Long Beach city manager Charles Theofan said high waves caused "considerable" amount of beach erosion, but water in the town was quickly receding.

The director of the Federal Emergency Management System (FEMA), Craig Fugate, said the U.S. East Coast power outages could last for days, depending on the amount of damage.

New York turned eerily quiet as the city hunkered down before the storm hit, crippled after the entire transit system was shut down because of weather for the first time in history. All the city's airports were closed, with over 9,000 flights cancelled. Broadway shows, baseball games and other events were all cancelled or postponed.

Forecasters said there was a chance a storm surge on the fringes of Lower Manhattan could send seawater streaming into the maze of underground vaults that hold the city's cables and pipes, knocking out power to thousands and crippling the nation's financial capital.

Gasoline supplies were falling as drivers fill up before leaving town or just top off their tanks as a precaution before the storm hits. Pump prices rose about three cents per gallon overnight in New Jersey and Pennsylvania.

New York's biggest utility, Consolidated Edison, said it could cut power to the city's most vulnerable areas if the storm causes serious flooding. Salt water and rain can damage electrical equipment.

N.Y. Stock Exchange expected to open Monday

The New York Stock Exchange has backup generators and can run on its own, a spokesman said. The exchange expects to open as usual Monday morning, though it may change plans depending on the severity of the storm.

People run from the beach in Kitty Hawk, Outer Banks, N.C., Saturday, as Hurricane Irene reaches the North Carolina coast. Charles Dharapak/Associated Press

ConEd has called in crews from as far as Colorado to help repair damage from the storm.

Power companies in affected areas have called in several hundred workers from surrounding states to help. Crews were rushing out between bands in the hurricane, when the wind and rain eased. They were looking for damage first at towering transmission lines, where an outage could put an entire county in the dark.

The storm caused gasoline supplies to fall as refuelling barges waited out the storm off the coast. Widespread power outages could lead to fuel shortages because of disabled gas pumps and trouble replenishing gas supplies.

Some gas stations in New Jersey reported that they'd run out of fuel. Those shortages could become more widespread.

Retail gas prices were mostly unchanged in many cities that are expected to be hit this weekend. Rules against price gouging at gas stations took effect throughout Middle Atlantic states. Authorities will be looking for stations that try to take advantage of panicked drivers.

With files from The Associated Press