GUANGZHOU, China — Authorities in the southern Chinese province of Guangdong are carrying out mass testing and have locked down areas to try to control a flare up of coronavirus cases in Guangzhou.

The city has cited the Delta variant of the coronavirus, first detected in India, as a driver behind the uptick in cases it has reported since the latter part of May. The Delta strain is known to be highly transmissible.

Guangzhou, a city of over 15 million people and the provincial capital, has reported 96 of the over 100 cases in Guangdong province in this latest outbreak.

China, where the coronavirus first emerged last year, quickly got the epidemic under control and has had very few cases over the past 12 months. However, clusters have emerged in parts of the country including major cities like capital Beijing and finance hub Shanghai.

The Guangzhou cases are potentially even more concerning because they involve the Delta strain of the coronavirus, which can spread very quickly.

A 75-year old woman in Liwan, a district of Guangzhou in the west of the city, was the first confirmed case of the Delta variant on May 21. She visited a restaurant and ended up infecting her husband. The latest infections began from there and has since spread to other areas of the city.

Liwan, still the worst-hit district, has imposed strict lockdowns on certain streets. Some areas are not letting people in an out of a certain zone and residents are not allowed to leave their building. Twenty-four hour checkpoints have been set up to monitor movement in and out of these areas.

Restaurants and entertainment venues have also been shut down.

But the virus has also spread to other parts of the city and province, too. Foshan, a city just southwest of Guangzhou has reported cases. On June 4, six members of the same family in the Nansha district of Guangzhou tested positive for the coronavirus. On Sunday, a positive case was found in the Chinese technology hub of Shenzhen, home to companies including Huawei and Tencent.

In other areas of Guangzhou less affected by latest cluster of cases, some restaurants and bars have begun offering takeout menus.

After the first case was found, Guangzhou initially carried out mass testing in Liwan that has since expanded to other areas.

In the central business district, known as Zhujiang New Town, residents were asked to take a test at a site near their apartments between Friday and Sunday.

One such testing site, which was set up on a road full of bars and restaurants, had huge lines on Friday.

Guangzhou carried out over 16 million tests between May 26 and midnight on June 5.

In Guangzhou, authorities have imposed tighter travel restrictions. Some metro stations in the city are closed. Authorities have urged people to not leave the city. But if residents need to leave the province, they should have a negative nucleic acid test conducted within 48 hours of their departure. Previously, travelers had a 72 hour window.

Hundreds of domestic flights have also been canceled from Guangzhou’s Baiyun International Airport.

Guangzhou has become a hub for driverless car companies to test their vehicles on public roads. And with Liwan locked down, these firms are transporting goods with their autonomous vehicles to Liwan.

Guangzhou-headquartered company WeRide has used its autonomous bus to transport food to Liwan. Pony.ai, another autonomous driving firm, has sent its vehicles to Liwan with supplies.

Chinese internet giant Baidu also used its autonomous vehicles to deliver food and medical staff to affected areas.