Hurricane Irene continues to pound the U.S. east coast with heavy rains and strong winds, leaving at least four people dead in its wake and paralyzing ground and air traffic.

Irene blasted ashore in North Carolina early Saturday, flooding streets and toppling trees with sustained winds of 140 kilometer per hour. Almost one million homes and businesses are without power.

U.S. officials say three people were killed in North Carolina and one in Virginia.

The storm has been downgraded to a Category One hurricane and forecasters say it will continue moving north-northeast at about 24 kilometers per hour. Despite weakening, Irene is still expected to pass through some of the country's most densely populated areas — including New York, Washington, Philadelphia and Boston.

Earlier Saturday, President Barack Obama visited the command center where senior emergency officials have been coordinating the country's response. He warned the situation would be critical for the next 72 hours.

Forecasters say hurricane-force winds are being felt up to 150 kilometers away from the eye of the storm. They said there is also a danger of tornadoes forming along the outer edge of the storm.

The U.S. Navy ordered the Second Fleet, stationed in Virginia, out to sea to avoid potential damage.

In New York, Mayor Michael Bloomberg has ordered the city's first ever mandatory evacuation in low-lying areas of the city, including the Wall Street financial district. And in another first, New York's entire public transit system, including subway trains and buses, began shutting down at midday, as well as all three of New York's major airports.

Airlines have already canceled thousands of flights up and down the east coast, and train service in parts of the eastern region have been suspended.

The Red Cross said its response to Irene could be one of the largest it has undertaken in recent memory. The organization is responding to more than a dozen states, and says it could take weeks, even months, to be able to fully address the disaster.

Irene is the first hurricane to seriously threaten the United States in three years. Besides the U.S. fatalities, Irene has killed at least one person in Puerto Rico and two in the Dominican Republic, and destroyed homes in The Bahamas.

It came almost six years to the day after Hurricane Katrina flooded New Orleans, killing more than 1,800 people and forcing more than a million residents from their homes.