BEIJING — At least nine people have tested positive for COVID-19 in a Chinese city on the border with Myanmar, health officials said Wednesday.

Five are Chinese citizens are four are Myanmar nationals, the Yunnan Province Health Commission said in a report posted online.

The city of Ruili, with a population of about 210,000 people, said all residents would be tested for COVID-19 and would have to home quarantine for one week. The residential compound where the infections were found has been locked down.

The city also ordered a crackdown on people who cross the border illegally, anyone who shelters them and those who organize such border crossings. It wasn’t immediately clear how the outbreak started.

The government has stepped up border control efforts since the coronavirus outbreak to try to stem the flow of people in both directions. Residents told the AP that government workers have been recruited to do monitoring shifts along the border.

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A military and police crackdown on protesters in Myanmar may be driving more people across the border, though it is difficult to gauge. Border control is considered an important part of China’s coronavirus prevention efforts.

China has largely eradicated the spread of the coronavirus and takes strict measures whenever a new cluster emerges. Ruili has closed all businesses except supermarkets, drugstores and food markets. Checkpoints were set up to limit entry to epidemic-related vehicles.

The first case was identified Monday, and subsequent testing of close contacts and others turned up the other ones, the Yunnan Health Commission said.

Three of the infected people, all Myanmar nationals in their 20s, did not have any COVID-19 symptoms. Of the others, four Chinese and one Myanmar citizen were classified as mild cases, and the remaining Chinese person as a regular case. They ranged in age from 22 to 42 years old.

Ruili, a gateway for China-Myanmar trade, had a COVID-19 scare last September when two people who had crossed illegally from Myanmar tested positive, according to state media reports. A commentary in the official Xinhua News Agency on Wednesday asked if lessons had been learned from that incident or whether there are still loopholes.