The United States declared a swine flu outbreak a public health emergency as officials confirmed 20 cases in five US states and warned that they expected more in the coming days.

President Barack Obama is monitoring the spreading virus and has reviewed US capabilities to counter the deadly flu outbreak, which has killed up to 81 people in Mexico, White House homeland security advisor John Brennan told reporters.

Obama has ordered a "very active, aggressive, and coordinated response," Brennan said.

Richard Besser, the acting head of the Centers for Disease Control and Prevention (CDC), told a White House press conference that there were eight confirmed US cases in New York City, seven in California, two in Texas, two in Kansas and one in Ohio.

"As we look for cases of swine flu, we are seeing more cases of swine flu. We expect to see more cases of swine flu," said Besser.

"We’re responding aggressively to try and learn more about this outbreak" and to implement measures to control its spread, he added.

"We’ve ramped up our surveillance around the country to try and understand better what is the scope, what is the magnitude of this outbreak."

Although there the government has not issued a warning against travel to Mexico, Besser said warnings could be increased "based on what the situation warrants."

Homeland Security Department Secretary Janet Napolitano said the US government would officially declare a public health emergency later on Sunday in response to the outbreak, adding that the declaration was "standard operating procedure."

The move allows government agencies to free up federal, state and local agencies and their resources in preventing the spread of the virus.

The declaration also allows officials to use medication and diagnostic tests and releases funds to purchase additional antiviral medication.

"All persons entering the United States from a location of human infection of swine flu will be processed through all appropriate CDC protocols," she added.

Suspected swine flu cases were being tested in Europe, the Middle East and Asia, after the new strain gain attention out of Mexico last week.

Health officials in Canada have reported a total of six confirmed cases of the potentially deadly virus.

World Health Organization (WHO) officials warned that the new strain, apparently born when human and avian flu viruses infected pigs and became mixed, could further mutate.

US immigration officials are looking for people with flu symptoms, said Napolitano.

"Travelers who do present with symptoms, if and when encountered, will be isolated per established rules," she said.

"They will be provided both with personal protective equipment and we’ll continue to emphasize universal hand washing."

Similar emergency health declarations were issued for floods in recent months in the US states of Minnesota and North Dakota and President Barack Obama’s inauguration in January.

Napolitano said the government intends to release a quarter of the national stockpile of 50 million doses of the antiviral drugs Tamiflu and Relenza. The drugs would be made available to all US states, especially those with confirmed cases of the flu.

The Pentagon, she added, "has procured and strategically pre-positioned" seven million treatment courses of the flu drug Tamiflu.

Meanwhile the Chinese government banned hog and pork product imports from Mexico and parts of the United States with immediate effect in a bid to prevent swine flu from entering China, the agriculture ministry and the General Administration of Quality Supervision, Inspection and Quarantine said in joint statement.

Imports from the US states of Texas, Kansas and California were affected by the move, according to the statement.

The disease cannot be transmitted by eating properly cooked pork but the Philippines has already brought in a similar ban.

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