A novel coronavirus outbreak that began in the Vietnamese city of Da Nang more than a week ago has spread to at least four city factories with a combined workforce of about 3,700, state media reported on Monday.

Four cases were found in the plants of different industrial parks in the central city which employ a total of 77,000 people, the Lao Dong newspaper said.

Vietnam, praised widely for its decisive measures to combat the coronavirus since it first appeared in late January, is battling new clusters of infection having gone more than three months without detecting any domestic transmission.

Authorities on Monday reported one new case linked to Da Nang, a famous tourist attraction where a case was detected on July 24, Vietnam's first domestically transmitted case in 100 days.

The source of the new outbreak is unclear but it has spread to at least 10 places, including the capital, Hanoi, and the business hub of Ho Chi Minh City, infecting 174 people and killing six.

Twenty-three percent of the latest infections are asymptomatic, the government said in a statement.

The country of 96 million has confirmed 621 infections, with six deaths.

Prime Minister Nguyen Xuan Phuc said on Monday that early August was a "decisive time" to contain the new outbreak, which could have a more "critical impact" than previous waves of infection.

Vietnam has carried out 52,000 tests for the coronavirus in the past seven days, according to a Reuters analysis of official data.

The government said on Saturday it planned to test Da Nang's entire population of 1.1 million people, part of "unprecedented measures" to fight the pandemic. The city imposed a lockdown last week, closing entertainment venues and banning movement in and out of the city.

Authorities said on Sunday that the strain of virus detected in Da Nang was a more contagious one, and that each infected person could infect five to six people, compared with 1.8-2.2 for infections earlier in the year.