Drug-resistant tuberculosis is spreading, according to findings published on Friday that show the difficult-to-treat strains are often carried by travellers and immigrants.

The researchers found that the proportion of cases resistant to any first line drug had increased to 7.9 percent from 5.6 percent between 1998 and 2005. There was also a 0.9 percent increase in multi-drug resistance.

"The observed increases highlight the need for early detection, rapid testing of susceptibility to drugs, and improved treatment completion," Michelle Kruijshaar, a researcher at Britain's Health Protection Agency, and colleagues wrote in the British Medical Journal.

The rate of tuberculosis incidence fell slightly worldwide for a second straight year in 2006, but there were still 9.2 million new cases and the disease killed 1.7 million people, the World Health Organisation said in March.

The data came out a month after another WHO report showed that TB cases that defy existing drugs were occurring globally at the highest rates ever, with nearly 490,000 cases in 2006. Parts of the former Soviet Union were particularly vulnerable.

TB is an infectious bacterial disease typically attacking the lungs. The emergence and spread of drug-resistant germs makes treating it much harder and could make it even deadlier.

The British researchers looked at 28,620 confirmed TB cases between 1998 and 2005 and found that the level of multi-drug resistance stemmed more from problems of patient management than transmission within Britain.

They also said there were 8,000 reported tuberculosis cases in England, Wales and Northern Ireland in 2006, an increase possibly due to a rise of the disease among immigrants from Sub-Saharan Africa and the Indian subcontinent.

A Norwegian study earlier this year found that immigrants with TB are not likely to spread the disease in their new countries because they often keep to within their own groups.

Other researchers said better efforts to catch the disease in marginalised populations is key to stamping out the disease and forms that are resistant to drugs.

"Clearly there's the concern of spread in the population, but in our experience, much of the resistant tuberculosis is found in patients who come from abroad, emphasising the need for screening at the earliest possible opportunity," said Geoffrey Pasvol, researcher at Imperial College London.