WHO Director General Maragaret Chan has warned that emerging, hard-to-treat strains of tuberculosis could spiral out of control.

Speaking in Beijing to health ministers from the 27 countries worst-affected by the new drug-resistant strains of TB, Chan urged ministers to improve methods of detecting infection and build stronger health care systems.

Threat greatest to developing countries

The drug resistant strains are spreading fastest in developing countries that lack the infrastructure to tackle the disease, Chan warned.

More than half of new TB cases are resistant to multiple drugs right from the start, she said.

"This is the true alarm bell. This tells us that resistant strains are now circulating in the general population, spreading widely and largely silently in a growing pool of latent infection," Chan said.

"Obviously this is a situation set to spiral out of control. Call it what you want, a time bomb or a powder keg, any way you look at it this is a potentially explosive situation," she said.

According to WHO data from 2006, there are nine million new TB cases annually. About 500,000 cases are classified as multiple-drug resistant (MDR-TB), while about 40,000 involve the more serious extensively drug resistant strain (XDR-TB).

China, Bill Gates foundation cooperate

People with XDR-TB, which has cropped up in 55 countries, have few treatment options and death rates are high.

In a spur to action, software magnate Bill Gates' foundation and the Chinese government have announced a $33 million project for new treatments and better ways to track patients.

China ranks second among countries with high rates of MDR-TB, just after India.