The highly contagious Delta variant of Covid-19 is expected to become the dominant strain of the virus over the coming months, the World Health Organization said Wednesday.

Delta, which was first detected in India, has now been recorded in 124 territories -- 13 more than last week -- and already accounts for more than three-quarters of sequenced specimens in many major countries, the WHO said.

"It is expected that it will rapidly out-compete other variants and become the dominant circulating lineage over the coming months," the UN health agency said in its weekly epidemiological update.

Of the three other coronavirus variants of concern (VOCs), Alpha, first detected in Britain, has been reported in 180 territories (up six from last week), Beta, first detected in South Africa, in 130 (up seven) and Gamma, first detected in Brazil, in 78 (up three).

According to SARS-CoV-2 sequences submitted to the GISAID global science initiative over the four weeks to July 20, the prevalence of Delta exceeded 75 percent in several countries.

Those included Australia, Bangladesh, Botswana, Britain, China, Denmark, India, Indonesia, Israel, Portugal, Russia, Singapore and South Africa.

"Growing evidence supports the increased transmissibility of the Delta variant as compared to non-VOCs. However, the exact mechanism for the increase in transmissibility remains unclear," said the WHO.

- Cases up 12% -
The Geneva-based organisation said overall, 3.4 million new Covid-19 cases were reported in the week to July 18 -- up 12 percent on the week before.

"At this rate, it is expected that the cumulative number of cases reported globally could exceed 200 million in the next three weeks," said the WHO.

The organisation said the global increases in transmission appeared to be driven by four factors: more transmissible variants; the relaxation of public health measures; increased social mixing and large numbers of unvaccinated people.

Cases were up 30 percent in the WHO's Western Pacific region and up 21 percent in its European region.

The highest numbers of new cases were reported from Indonesia (350,273 new cases; up 44 percent), Britain (296,447 new cases; up 41 percent), and Brazil (287,610 new cases; down 14 percent).

The number of weekly deaths, however, remained steady at 57,000, similar to the previous week and following a steady decline for more than two months.