U.S. President Barack Obama on Sunday warned that flooding from Hurricane Irene could worsen as rivers flood their banks and said federal recovery efforts would last a few weeks.

I want people to understand this is not over, Obama said in an appearance at the White House. Response and recovery efforts will be an ongoing operation.

Irene was downgraded to a tropical storm on Sunday and was bringing heavy rains and wind to New England as it headed north. The storm forced transit closures and flight cancellations and led to widespread power outages along the East Coast.

But New York City appeared to have avoided the mass devastation that many had feared.

One of our chief concerns before Irene made landfall is the possibility of significant flooding and widespread power outages and we've been getting reports of just that from our state and local partners, said Obama, who was flanked by Secretary of Homeland Security Janet Napolitano and Craig Fugate, head of the Federal Emergency Management Agency.

Many Americans are still at serious risk of power outages and flooding, which could get worse in the coming days as rivers swell past their banks, he added.

Obama cut short his vacation in Martha's Vineyard in Massachusetts by one day to return to Washington on Friday to oversee preparations for the storm.

He led a video conference with top aides on the response on Sunday morning and the White House said another call was planned for Sunday evening.