"This outbreak is at a highly alarming level because of multiple transmission sources and different strains of viruses that allow rapid infection, which make it difficult to control," he said at a meeting Friday.

A fourth wave of new coronavirus began on April 27 when the first case was confirmed.

Since then at least five hotspots have been identified, mostly in the north, and the infection has spread to 19 cities and provinces, including Hanoi and HCMC.

Worryingly, the sources of transmission for the outbreaks at Hanoi’s frontline Covid medical facility, the National Hospital for Tropical Diseases, and Da Nang are still unclear.

Most patients have been infected with the U.K. and Indian strains, which experts worldwide have said are "more concerning" due to their ability to spread much faster than those found in 2020.

It is highly possible that more hotspots would appear with new sources of transmissions, Long warned.

"This is why mass testing is vital as it allows early detection of infections to locate the affected areas and contain the spread as soon as possible.

"It is the only way."

Deputy Prime Minister Vu Duc Dam, head of the National Steering Committee for Covid-19 Prevention and Control, said at the meeting, "It is possible that the transmission source could have already existed within the community."

As of Saturday morning 176 people have been found infected. Hanoi leads with 72 cases, followed by the northern Vinh Phuc and Ha Nam Provinces with 26 each.