China has placed millions under lockdown as the nation faces the biggest outbreak of the coronavirus in months due to the more transmissible delta variant.

Initial reports from last week noted that the variant had spread to 15 cities, but that number had risen to 20 cities by Monday. The latest outbreak started July 20 with the variant detected on a plane that arrived at Nanjing Lukou International Airport during a routine cleaning.

The infection resulted in 184 new infections since then, of which 52 were linked to the delta variant, Reuters reported.

A worker transporting work tools pass by security personnel outside the Tianjin Binhai No. 1 Hotel where U.S. and Chinese officials are expected to hold talks in Tianjin municipality in China on Sunday, July 25, 2021. Deputy Secretary of State Wendy Sherman travelled to China this weekend on a visit that comes as tensions between Washington and Beijing soar on multiple fronts, the State Department said Wednesday. (AP Photo/Ng Han Guan)

"After their work was complete, due to cleaning and protective measures not meeting standards, it's possible some staffers got infected, causing the virus to spread among cleaning staff," Ding Jie, deputy director general at the Nanjing Center for Disease Control and Prevention, said.

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Ding said that new cases are "constantly being discovered," the South China Morning Post reported.

The delta variant has shown to be far more transmissible than other strains of COVID-19: Centers for Disease Control Director Rochelle Walensky claimed in mid-July that the variant is "spreading with incredible efficiency and now represents more than 83% of the virus circulating the United States."

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As such, China has moved swiftly to contain the spread of the new strain and any possible cases. All flights from Nanjing have been cancelled until Aug. 11, but that didn’t stop the spread within the country itself.

Local governments in major cities, including Beijing, have tested millions of residents while maintaining strict quarantine measures on any potentially infected areas.

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Beijing has cut all rail, bus and air travel with areas that have detected new cases. Only "essential travelers" are allowed to enter as long as they provide a negative nucleic acid test, The Guardian reported.

"The city is still grim and complicated," officials in the central city of Zhuzhou in Hunan province said Monday as it ordered a strict three-day lockdown for more than 1.2 million residents.

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New cases were found Monday in popular tourist destination Hainan, indicating the nation’s problem will only grow more severe in the coming days.