Authorities in China cancelled hundreds of flights, closed schools and ramped up mass testing on Thursday to try and stamp out a new Covid-19 outbreak linked to a group of tourists.

Beijing has maintained a relentless zero-Covid approach with strict border closures and targeted lockdowns, even as other countries tentatively try to ease restrictions.

Domestic outbreaks have largely been eliminated, but as China logged a fifth straight day of new cases — mostly in northern and northwestern areas — authorities beefed up coronavirus controls.

The latest outbreak was linked to an elderly couple who were in a group of several tourists. They started in Shanghai before flying to Xi’an, Gansu province and Inner Mongolia.

Dozens of cases have since been linked to their travel, with close contacts in at least five provinces and regions, including the capital Beijing.

In response, local governments have rolled out mass testing and closed scenic spots and tourist sites, schools and entertainment venues in affected areas, and also imposed targeted lockdowns of housing compounds.

Some regions including Lanzhou — a city of some four million people in northwestern China — have told residents not to leave unless necessary.

Those who need to leave must present a negative Covid-19 test.

Airports in the affected regions have cancelled hundreds of flights, according to data from aviation tracker VariFlight.

Around 60 percent of flights to the two main airports in Xi’an and Lanzhou have been cancelled.

In a notice published Monday, Erenhot in Inner Mongolia said travel in and out of the city was banned and residents should not leave their housing compounds.

And on Wednesday, state-owned tabloid Global Times cautioned that the new virus cases in Inner Mongolia were likely to affect coal imports from Mongolia because of supply chain disruptions.

There were 13 new domestic cases reported on Thursday, China’s National Health Commission said.