London ? Drug-resistant tuberculosis is spreading even faster than medical experts had feared, the World Health Organization warned in report issued Tuesday.

The rate of TB patients infected with the drug-resistant strain topped 20 percent in some countries, the highest ever recorded, the U.N. agency said.

“Ten years ago, it would have been unthinkable to see rates like this,” said Dr. Mario Raviglione, director of WHO’s “Stop TB” department. “This demonstrates what happens when you keep making mistakes in TB treatment.”

Though the report is the largest survey of drug-resistant TB, based on information collected between 2002 and 2006, there are still major gaps: Data were only available from about half of the world’s countries.