The US East Coast has begun clearing up after the devastation of tropical storm Irene, which killed at least 21 people. The storm is now lashing Canada\'s north-east, after causing severe flooding in the US and leaving some five million homes without power. But New York was not nearly as badly affected as state officials had feared. President Barack Obama has warned that the impact of the storm will be felt for some time and that the recovery effort will last for weeks. Flooding and power cuts are still a risk as swollen rivers could burst their banks, he said on Sunday. The brunt of Irene\'s impact was felt by towns and suburbs from New Jersey to Vermont. Driving rains and flood tides damaged homes and cut power to more than three million people in New Jersey, Connecticut and New York. The BBC\'s Steve Kingstone in New York said insurance and rebuilding costs would run into billions of dollars. Irene was earlier downgraded to a tropical and then a post-tropical storm. More than 300,000 people evacuated from low-lying areas in New York City are heading home. New Yorkers will attempt to return to work on Monday, but the subway service will be limited while the tracks are inspected, says the BBC\'s Laura Trevelyan in New York. Most of the commuter rail services feeding the city were out indefinitely, reports say. The New York Stock Exchange said it would be open for business on Monday and officials at the 9/11 memorial at the World Trade Center site said they had not lost a single tree. Airlines said about 9,000 flights had been cancelled, but services into New York and Boston were due to resume on Monday. In Philadelphia, officials lifted the city\'s first state of emergency since 1986. Several buildings were destroyed by the storm, but there were no deaths or injuries. Vermont lashed Widespread flooding is reported in Vermont where hundreds of people have been told to leave the capital, Montpelier. President Obama: \"The impact of the storm will last for some time\" The city faces flooding, once from Irene, and again if the local water company decides to release water to save the Marshal Reservoir, a local dam where waters are reaching record levels. \"It\'s very serious for us at the moment in Vermont. The top two-thirds of the state are inundated with rapidly rising waters, which we anticipate will be an issue for the next 24 hours,\" said Robert Stirewalt, a spokesman for Vermont Emergency Management Agency. Further south in North Carolina, Governor Beverly Perdue said some areas of the state were still unreachable. TV footage showed fallen trees and power lines. Officials in Virginia began the clear-up, but said the damage was not a bad as feared. The north-eastern seaboard is the most densely populated corridor in the US. More than 65 million people live in major cities from Washington DC in the south to Baltimore, Philadelphia, New York and Boston further north. Irene was classified as a category-three hurricane, with winds of more than 120mph (192km/h), when it swept through the Caribbean last week.