Obama says Hurricane Irene 'extremely dangerous'

President Barack Obama on Friday warned Americans to take Hurricane Irene seriously and urged them to obey orders to evacuate from the path of what is likely to be an "extremely dangerous and costly" storm.

"All indications point to this being a historic hurricane," Obama said in a statement to reporters from the farm where he is vacationing on this island off the Boston coast.

Fifty-five million people are potentially in Hurricane Irene's path, from the Carolinas to Cape Cod on the US East Coast, and tens of thousands are evacuating as cities including New York and Washington brace for the powerful storm to hit.