China is conducting mass testing in Wuhan after it identified its first domestic cases of the Delta variant in the city where coronavirus was first detected in late 2019.
But on Monday, authorities confirmed three new cases of the more transmissible variant in the Chinese city.
All of its 12 million residents will be tested for coronavirus.
"To ensure that everyone in the city is safe, city-wide nucleic acid testing will be quickly launched for all people to fully screen out positive results and asymptomatic infections," said Li Qiang, an official in the city, the capital of central Hubei province.
The new cases in Wuhan, along with infections in the nearby cities of Jingzhou and Huanggang since Saturday, were linked to cases found in Huaian city in Jiangsu province, said Li Yang, vice director of Hubei's provincial disease control centre.
The outbreak in Jiangsu is believed to have begun in the provincial capital of Nanjing, with the Delta variant mostly likely introduced on a flight from Russia, officials have said.
Since then numerous cities in southern China and a few in the north including Beijing have reported infections.
The number of locally transmitted cases in China since 20 July, when the first Nanjing infections were found, stood at 414 on Monday.
However, it is not clear if all of those cases were of the Delta variant, or if they were all linked to Nanjing, as some authorities have not disclosed conclusive results of their virus-tracing efforts.
The Delta variant poses new risks to the world's second-largest economy as it spreads from the coast to inland cities.
On Tuesday, the National Health Commission said 90 new cases had been confirmed the previous day, 61 locally spread ones and 29 among people who had recently arrived from abroad.
Most of the local cases were in Jiangsu province, where an outbreak started at the airport in Nanjing, the provincial capital, and has spread to the city of Yangzhou, 65 miles away.
Authorities reported 45 new cases, five in Nanjing and 40 in the city of Yangzhou, which was conducting a second round of mass testing.
Five other provinces and the cities of Beijing and Shanghai reported new local cases in the single digits.
In Shanghai, the nation's largest city, a driver working at one of its two main airports tested positive. Beijing has reported a total of five cases in recent days.
Government-affiliated scientists have said Chinese vaccines are less effective against the new strains of the coronavirus but still offer some protection.
Only Chinese vaccines are currently being given in China, where authorities say more than 1.6 billion doses have been administered.