New York, Sep 8 (IANS) Scientists have found cases of virtually untreatable tuberculosis across the globe including in the US, Eastern Europe and Africa.

Researchers are worried about the emergence of TB strains that are resistant to drugs, reported the online edition of BBC News.

TB, a disease of the respiratory system spread by coughing and sneezing, causes about 1.7 million deaths a year worldwide.

Extreme drug resistant TB (XDR TB) is defined as strains that are resistant to not only frontline drugs but also three or more of the six classes of second-line drugs.

A recent survey of 18,000 TB samples by the US-based Center for Disease Control and the WHO between November 2004 and November 2005 found 20 percent of them were multi-drug resistant and a further two percent were extreme drug resistant.

Further detailed analysis of several countries found the prevalence was even higher.

In the US, four percent of all multi-drug resistant TB (MDR TB) cases met the criteria for XDR TB; in South Korea, the figure was 15 percent, according to WHO expert Dr Paul Nunn.

In Latvia and other areas of the Baltics and the former Soviet Union, 19 percent of all multi-drug resistant cases were extreme drug resistant too.

XDR TB was present across several strains, Dr Nunn said, but added it was not yet clear how transmissible it was or whether it was limited to isolated pockets.

"XDR TB is very serious - we are potentially getting close to a bacteria that we have no tools, no weapons against."

He added that it was key that new drugs were developed in future and said work was underway looking at new drugs, including research into TB vaccines.

TB experts have convened in Johannesburg, South Africa, to discuss how to address the problem.