UPDATE 1-U.S. acts swiftly to contain swine flu outbreak

* US to release 25 percent of anti-viral stockpile

* Health chief recommends planning for school closures

* Too early to determine impact on economy (Updates to add information from CDC briefing)

By Ross Colvin

WASHINGTON, April 26 (Reuters) - The United States declared a public health emergency on Sunday because of an outbreak of swine flu that has been diagnosed in 20 people in this country -- the same strain suspected of killing 81 people in Mexico.

The outbreak is yet another distraction for President Barack Obama as he focuses on rescuing the economy from its worst crisis in decades. His administration will also be mindful of the damage to former President George W. Bush over his government's inept handling of Hurricane Katrina in 2005.

"At this point, a top priority is to ensure that communication is robust and that medical surveillance efforts are fully activated," John Brennan, assistant to the president for Homeland Security, told a White House briefing.

Dr Anne Schuchat of the Centers for Disease Control and Prevention told a separate briefing she feared that some people would die in the United States as the virus spread.

Health and Homeland Security officials announced steps to release some of the U.S. stockpiles of the anti-flu drugs Tamiflu and Relenza. They recommended that local authorities plan for possible school closures and that anyone with symptoms stay at home to reduce the possibility of transmission.