"The 2009 H1N1 pandemic continues to evolve. The rates of illness continue to rise rapidly within many communities across the nation, and the potential exists for the pandemic to overburden health care resources in some localities," Mr Obama said in a statement.

"Thus, in recognition of the continuing progression of the pandemic, and in further preparation as a nation, we are taking additional steps to facilitate our response."

The president signed the declaration late on October 23 Friday and announced it on October 24.

Since the H1N1 flu pandemic began in April, millions of people in the United States have been infected, at least 20,000 have been hospitalized and more than 1,000 have died.

16.1 million doses of H1N1 vaccine had been made by Friday -- 2 million more than two days earlier. About 11.3 million of those had been distributed throughout the United States.