Richard Besser, acting director of the federal Centers for Disease Control and Prevention, said deaths are likely.
"I fully expect we will see deaths from this infection," he said, quoted the Associated Press.
Listen to Besser's update during Tuesday's CDC press conference on the swine flu:
Besser said the CDC was aggressively investigating the disease's spread and looking for ways to control and prevent it.
Homeland Security Secretary Janet Napolitano said in network television interviews that the United States has issued a "very broad multi-agency federal response," and that the administration wouldn't wait for a World Health Organization declaration of a pandemic to deliver a pandemic-like response.
"We're prepared as if there were a pandemic. We're not waiting," she said, according to the AP.
In Mexico, the swine flu outbreak has caused more than 150 deaths. Asian countries have deployed thermal sensors at airports to screen passengers from North America for signs of fever.
Health officials also confirmed new cases in New Zealand, Canada, Britain, Spain and Israel on Tuesday.
Of the 64 cases in the United States, 45 are in New York, one in Ohio, two in Kansas, six in Texas and 10 in California, reported the AP.
The CDC has issued an advisory warning to travelers to cancel any nonessential visits to Mexico, while European Union health officials issued the same warning about traveling to parts of the United States.
The World Health Organization raised its alert level Monday to indicate a significantly increased risk of pandemic, or global outbreak of serious illness, but did not move to recommend travel restriction and border closures.
On Tuesday, Napolitano said that federal efforts to get antiviral medications to the states "is under way and is working."
The Food and Drug Administration, for example, issued emergency guidelines late Monday that allow certain antiviral drugs to be used in a broader range of the population in case mass dosing is needed to deal with a swine flu outbreak, according to the AP.
The agency originally approved the use of the antiviral drug Tamiflu for the prevention and treatment of influenza in adults and children age 1 and older. Another antiviral drug, Relenza, was approved to treat people 7 and older and to help prevent flu in those 5 and older.
Numerous companies around the world are implementing travel restrictions in response to the flu outbreak, including Nokia.
"Yesterday we gave new travel instructions because of the swine flu. The new guidance is that all non-essential travel from and to Mexico should be postponed for the time being," said Finland's Nokia, the world's largest mobile handset maker, which employs 3,500 in Mexico, mostly at a plant in Reynosa, Reuters reported.
Honda Motor Co., Japan's No.2 automaker, has suspended all global business travel until at least May 6 due to the outbreak, and other companies are taking similar precautions.