WASHINGTON, Jan 29 (Reuters) - U.S. regulators warned Novartis AG <NOVN.VX> that a German plant has produced contaminated vaccines, and it has failed to track down the source of the problems, a letter released on Tuesday said.

The U.S. Food and Drug Administration found significant deviations from good manufacturing practices at the drugmaker's Marburg, Germany plant, which makes a rabies vaccine, and diphtheria and tetanus concentrates, according to Jan. 24 letter posted on the FDA's Web site on Tuesday.

The FDA letter cited multiple cases of contamination and non-sterility, and failure to test and track down the source of these cases.

The agency gave the company 15 days to respond to the letter.

A spokeswoman for Novartis was not available for comment.

The FDA sends dozens of warning letters each year and most are resolved without further action. In rare cases, the agency can hold up approvals or seize products.

(Reporting by Kim Dixon, editing by Richard Chang)

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