China has locked down a city on the border with Myanmar and ordered all its residents to be tested for Covid-19 after a number of people infected with the coronavirus illegally entered the city from the neighbouring country.

Official media reported that “wartime” anti-epidemic measures have been put in place in Ruili city with over 200,000 people in southwest China’s Yunnan province after three coronavirus infections were detected.

All Ruili residents have been directed to stay indoors as local authorities are expected to start mass nucleic acid tests for them.

Screening for illegal immigrants from Myanmar will be carried out in communities, shops, restaurants and construction sites.

Transport connections have been snapped and residents have been ordered not to leave the city until further notice. Outsiders are not being allowed inside city limits.

Trade and businesses, except those dealing in essential services such as pharmacies and food markets, have been closed.

The infections travelled from Myanmar and Chinese authorities would “crack down on illegal immigrants”, local officials said late on Monday.

Besides Ruili, at least eight border prefectures and 25 border counties in the province should enter wartime status immediately, enhancing anti-epidemic measures and border management to prevent imported cases, local officials were quoted as saying by official media.

Chinese media reports said authorities were mobilising residents to report “any illegal border crossings in a timely manner, stating that assistance in any form, including the provision of shelter for illegal trespassers, is strictly prohibited”.

News agency AFP quoted Yang Bianqiang, vice mayor of Ruili, as saying that the city would repatriate those who cannot verify their time of arrival into China, referring to people who “have no fixed residence and have no fixed place to work”.

Beijing had shut a border crossing with Myanmar in April after infighting in the country had spilled over into a Chinese province.

Myanmar is a vital part of China’s Belt and Road Initiative. It was the last country that Chinese President Xi Jinping had visited, in the third week of January, before the Covid-19 outbreak all but shut international travel out of China.

China has largely controlled the Covid-19 pandemic in the country except a number of localised outbreaks, including in Beijing, in recent months.

The coronavirus first emerged in the Chinese city of Wuhan last year, before triggering the pandemic and affecting economies around the world.

As of Monday, the total number of confirmed Covid-19 cases on the mainland reached 85,202. Altogether, 80,426 people have been discharged and 4,634 have died from the disease on the mainland, the national health commission said in its daily update on Tuesday.

Eight new imported Covid-19 cases were reported on Monday, taking the total number of imported infections to 2,653.