Every province has advised residents not to leave as flights are cancelled and Beijing suspends more than a dozen rail lines.

China has dramatically tightened travel restrictions as it seeks to control the country’s worst outbreak in months, with hundreds of Delta variant cases linked to airport employees.

The latest outbreak has so far infected more than 400 people in 25 cities, including the capital city Beijing, and in Wuhan for the first time since it contained the first Covid-19 outbreak last year.

Cases have been reported in 17 of the 31 provinces.

A further 71 locally transmitted cases were confirmed on Tuesday, the national health commission said Wednesday – the highest daily count since January.

Nearly half were in Jiangsu, the site of the airport cluster to which most cases have been linked, and 15 in Hunan.

The governments of all 31 provinces have advised residents to avoid leaving their regions unless necessary, and to stay away from the four high risk – and more than 120 medium risk – regions across China, in a bid to curb further transmission of the highly infectious Delta variant.

In addition to various lockdown measures, Nanjing and Yangzhou have since cancelled all domestic flights, while Beijing has suspended 13 rail lines and halted inbound long-distance ticket sales from 23 stations, according to state-run news agency Xinhua.

Yangzhou, Wuhan, and the flood-hit city of Zhengzhou have launched citywide testing and Zhengzhou now requires all people to show a negative test result in order to leave the city.

Residential areas, including those home to more than 10,000 people in Beijing, have been sealed off for mass testing.

Authorities have also begun testing all 11 million residents of Wuhan.

China’s top infectious diseases expert, Zhong Nanshan, said most Delta patients had shown mild symptoms and preliminary studies indicated China’s vaccines were effective in reducing the severity of the variant, Xinhua said.

While it has seen several outbreaks since mid-2020, Chinese authorities have contained them through ambitious mass testing drives of entire cities, strict localised lockdowns, and targeted travel restrictions.

However, the high transmissibility of the Delta variant has seen the number of cases rise rapidly and spread far.

Most cases have been linked to Nanjing and Lukou airport staff who cleaned an incoming plane from Russia, as well as domestic planes.

A cluster of infections among tourists who went to a concert in Zhangjiajie, Hunan, travelling through Lukou has also spread to multiple provinces.

“Zhangjiajie has now become the new ground zero for China’s epidemic spread,” Zhong said earlier this week.

In an editorial on Sunday, state tabloid the Global Times said China could not afford to make errors like those identified in Nanjing, given the high rates of infections around the world.

“The challenge for China is to open controllable windows between our closed anti-epidemic system and the turbulent outside world, which can not only guarantee the openness of Chinese society, but also maintain China’s capability of dynamically clearing Covid-19 cases,” it said.

There are also two other Delta outbreaks linked to Myanmar, including the border province Yunnan, and Zhengzhou which received air passengers from Myanmar.