The head of the World Health Organization, or WHO, warned that cases and deaths from COVID-19 have continued to rise, with almost 4 million cases reported to the organization last week.

It's expected the total number of cases will pass 200 million within the next two weeks, which the WHO said is an underestimate.

Speaking at a media briefing on Friday, Tedros Adhanom Ghebreyesus, director-general of the WHO, revealed that COVID-19 infections have increased by 80 percent in five of the organization's six regions, nearly doubling over the past four weeks.

"In Africa, deaths have increased by 80 percent over the same period," Tedros said, adding that the "highly-transmissible" Delta variant, which has now been detected in at least 132 countries, is the main cause for the increase.

"WHO has warned that the COVID-19 virus has been changing since it was first reported, and it continues to change. So far, four variants of concern have emerged, and there will be more as long as the virus continues to spread," he said.

He noted that the rise is also driven by increased social mixing and mobility, inconsistent use of public health and social measures, and inequitable vaccine use.

"Hard-won gains are in jeopardy or being lost, and health systems in many countries are being overwhelmed," he said. "The increased number of COVID-19 infections is creating a shortage of treatments, such as life-saving oxygen. Twenty-nine countries have high and rising oxygen needs, and many countries have inadequate supplies of basic equipment to protect frontline healthworkers."

Testing rates in low-income countries are now less than 2 percent compared wi high-income countries, which Tedros said is leaving the world blind to understanding where the disease is and how the virus is changing.

He stressed that without better testing rates the world will struggle to fight the disease on the front line and also mitigate the risk of new and more dangerous variants emerging.

Tedros called for stronger surveillance and more strategic testing to understand where the virus is, as well as to find out where public health interventions are most needed.

The WHO's goal remains to support every country to vaccinate at least 10 percent of its population by the end of September, at least 40 percent by the end of this year, and 70 percent by the middle of next year, but Tedros said those targets are still a long way off.

In the effort to develop more vaccines, the WHO has signed a letter of intent that sets out the terms of collaboration with the Medicines Patent Pool; Afrigen Biologics; the Biologicals and Vaccines Institute of Southern Africa; the South African Medical Research Council and the Africa Centres for Disease Control and Prevention.

The letter comes after the WHO announced the development of a technology transfer hub for mRNA vaccines in South Africa last month, as part of efforts to scale up production of vaccines.