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The case of a California teen with Tamiflu-resistant swine flu has prompted U.S. officials to increase their testing of swine flu infections. Tamiflu is the primary drug treatment for the new swine flu virus.

The 16-year-old San Francisco girl is only the third person in the world to be diagnosed with a swine flu strain resistant to Tamiflu. She was diagnosed June 11 and has since recovered, the Associated Press reported.

The two other cases involved patients in Denmark and Japan who were taking Tamiflu after coming into contact with people infected with swine flu. But the American girl hadn't taken Tamiflu, which means she was infected by an already-circulating Tamiflu-resistant strain.

U.S. health officials say they're not alarmed because they've been expecting to see some Tamiflu-resistant cases of swine flu, the AP reported. Resistance to the drug is seen in other types of flu viruses.

Cases of Tamiflu-resistant swine flu are likely to be sporadic, "but it's very important to monitor them," said Dr. Tim Uyeki, a flu expert at the U.S. Centers for Disease Control and Prevention.

The CDC has asked health departments across the country to submit more swine flu virus samples for testing, the APreported.