BEIJING (REUTERS) - The major manufacturing hub of Dongguan in China's most populous province of Guangdong launched mass testing on Monday (June 21) for the coronavirus and cordoned off communities after detecting its first infections in the current outbreak.

The Delta variant of Covid-19 has dominated infections in the provincial upsurge, the first time it has hit China. Seen by experts as highly transmissible, the variant was first identified in India.

Dongguan launched a citywide testing programme on Monday, following two cases reported since last Friday.

City authorities told residents not to leave, except for essential reasons.

Even then, those leaving must show negative test results within 48 hours of departure.

Entrances on highways to other cities were closed, while shuttle buses between airports in Guangzhou and Shenzhen, and a check-in terminal in Dongguan, were halted. Some museums and libraries in the city also closed to visitors.

Its factories are still running, however.

"(Workers) need to do Covid-19 tests, but it's not a prerequisite for them to be able to enter factories," said King Lau, who helps manage a metal coating factory.

"My staff will do (their Covid-19 tests) after work, although there will be long queues." Guangdong has reported 168 confirmed infections since May 21, with nearly 90 per cent of them in its capital, Guangzhou.

But the cases are few compared with the rest of the world and previous outbreaks in China.

The northeastern region had more than 1,150 infections from late December to early February in the worst domestic outbreak after that in the central city of Wuhan, where the virus emerged in late 2019.

But Guangdong, a key entry point for travellers and cargo, has not taken any chances.

Although its capital has reported no cases for two successive days, the province as a whole is still reporting new infections in the 31-day outbreak.

Strict disinfection and quarantine measures since May 21 have led to congestion of vessels waiting to berth in one of China's busiest container ports, Yantian International Container Terminal (YICT) in Shenzhen.

"The impact would be bigger than the Suez Canal incident," said Patrik Berglund, chief executive of Xeneta, an ocean freight rate benchmarking firm headquartered in Oslo.

Although 50 vessels were waiting outside the port, more than 160 were being affected, he added.

"We've seen exporters who cannot wait for the port congestion to ease turning to trucks to send the cargoes from China to Europe." Normal operations are expected to resume by end-June.

But even as congestion at Yantian eases, traffic at the Shezhen port of Shekou and the main Guangzhou port of Nansha remains high, shipping firm Maersk said on its website.

Chinese experts said Guangzhou's fight against the Delta variant served as a warning for other cities not to get too complacent.

China reported 17 new confirmed mainland infections on Sunday, down from 23 a day earlier, its health authority said on Monday.

One of the new cases one was a local infection in Dongguan, while the rest were imported cases, the National Health Commission said.

The number of new asymptomatic infections fell to 19 from 20 a day earlier. China does not classify them as confirmed cases.

The tally of infections in mainland China stood at 91,604, with the death toll unchanged at 4,636.

Guangdong has sped up its vaccination effort since the outbreak. By May 19, before any local cases were reported, it had administered 39.15 million doses.

By June 20, the figure was 101.12 million, meaning more than 60 per cent of its doses were injected over one month.