Some people have been forced to jump into the ocean to escape the blaze.
A mass evacuation effort for stranded visitors and residents is under way, with buses set to pick up people in Kaanapali, north of the town of Lahaina.
Visitors will be taken directly to the Kahului Airport and residents to a shelter in central Maui.
The disaster designation announced by Mr Biden clears the way for federal aid to help the affected communities.
It will allow affected individuals to apply for grants for temporary housing and home repairs, and allow business owners to apply for programs to recover from the disaster, the White House said in a statement.
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The US president expressed his condolences during a phone call to Hawaii Governor Josh Green.
US senator Brian Schatz said “winds are finally backing off,” but the “number of dead continues to rise,” during a visit to Maui to assess the damage.
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He did not give any further details on the number of casualties.
Thousands of residents have already fled their homes, while more than 11,400 tourists have been evacuated to nearby islands.
A further 1,500 visitors are expected to leave on Thursday as United Airlines and American Airlines have sent planes to evacuate passengers.