LONDON — Drug-resistant tuberculosis is spreading even faster than medical experts had feared, the World Health Organization warned in report issued yesterday.

The rate of TB patients infected with the drug-resistant strain topped 20% 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.

In Africa, where experts are particularly worried about a lethal collision between TB and AIDS, only six countries provided information.

“We really don’t know what the situation is in Africa,” Raviglione said. “If multi-drug resistant TB has penetrated Africa and coincides with AIDS, there’s bound to be a disaster.”

Raviglione said it was likely that patients — and even entire outbreaks of drug-resistant TB — were being missed.

Experts also worry about the spread of XDR-TB, or extensively drug-resistant TB, a strain virtually untreatable in poor countries. When an XDR-TB outbreak was identified in AIDS patients in South Africa in 2006, it killed nearly every patient within weeks. WHO’s report said XDR-TB has now been found in 45 countries. Globally, there are about 500,000 new cases of drug-resistant TB every year, about 5% of the 9 million new TB cases. In the United States, 1.2% of TB cases were multi-drug resistant. Of those, 1.9% were extensively drug-resistant.

The highest rates of drug-resistant TB were in eastern Europe. Nearly a quarter of all TB cases in Baku, Azerbaijan, were drug-resistant, followed by about 20% in Moldova and 16% in Donetsk, Ukraine, WHO said.

High rates of drug-resistant TB were also found in China and India, the world’s two most populous nations that together are home to half the world’s cases.

Drug-resistant TB arises when primary TB treatment is poor. Countries with strong treatment programs, like the U.S. and other Western nations, should theoretically have very little drug-resistant TB.

That is not the case in China, however, where the government says 94% of TB patients complete their first TB treatment.

“There’s a huge, gross discrepancy there if they are then reporting 25% of the world’s multi-drug resistant TB cases,” said Mark Harrington, executive director of Treatment Action Group, a public health think tank. “They are clearly nurturing a multi-drug resistant TB epidemic and failing to report XDR-TB at all.”

With growing numbers of drug-resistant TB patients, there is concern some national health systems will soon be overwhelmed. “We are totally off track right now,” said Dr. Tido von Schoen-Angerer, executive director of Medecins Sans Frontiere’s Campaign for Access to Essential Medicines. He said only 30,000 multi-drug TB resistant patients were treated last year.

Experts said new drugs are needed if the outbreak is to be curbed, along with new diagnostic tests to identify drug-resistant TB strains faster — current tests take about a month for results.

WHO said a new diagnostic test able to provide results within a day is being tried in South Africa and Lesotho. If successful, the test could be introduced across Africa in a few months, though new labs would be needed to run the tests. “Multi-drug resistant TB is a threat to every person on the planet,” Harrington said. “It’s not like HIV, where you are only infected through specific actions. TB is a threat to every person who takes a train or a plane.”