MIKE STOBBE, AP Medical Writer

ATLANTA — Health officials say four people in North Carolina have tested positive for a type of swine flu that's resistant to the drug Tamiflu.

It's the first cluster of that many cases seen in the U.S.

Health officials said Friday the four cases were reported at Duke University Medical Center in Durham over the past six weeks.

Tamiflu is one of two medicines that help against swine flu. Health officials have been closely watching for signs that the virus is mutating, making the drugs ineffective.

About 52 resistant cases have been reported in the world since April, including 15 in the U.S. so far. Officials with the U.S. Centers for Disease Control and Prevention say almost all the U.S. cases have been isolated.