The Salvation Army has mobilised teams of responders after Hurricane Irene caused widespread damage along the US’s densely populated east coast.

At least 21 people have been killed by the hurricane, which has been downgraded to a tropical storm.

Millions have been left without power and damage caused by the strong winds and widespread damage is estimated to be at least $8bn.

As the storm now moves north towards Canada, around 300,000 people evacuated from low-lying regions in New York City have been given the green light to return home.

There is some flooding in the city but damage was less than expected.

Although the hurricane was downgraded, US President Barack Obama warned Americans that there was still “serious risk” of power outages and flooding.

“I want people to understand that this is not over. Response and recover efforts will be an ongoing operation and I urge Americans in affected areas to continue to listen for the guidance and direction of their state and local officials.”

The Salvation Army USA said it had moved personnel and vehicles into strategic locations ahead of the hurricane’s landfall.

Its mobile feeding units are being used to provide food and drink, as well as spiritual and emotional care to emergency responders and other people affected by the storm.

Major George Hood, National Community Relations Secretary for The Salvation Army in the US said: “[We] are working closely with partner agencies to ensure we can provide a strong response to any impacts of the storm.”

The Salvation Army is already working to serve people affected by Hurricane Irene in Puerto Rico, providing support to the homeless, sick and disabled.

It has distributed food and water supplies in the Bahamas and provided ready-to-eat meals in the Turks and Caicos, which were also affected.