Ruili, a city in Yunnan province that lies on the border with Myanmar, will begin free mass nucleic acid testing of all residents after reporting two cases of COVID-19 on Sept 13 — both in illegal immigrants who arrived in the city recently — the local government said on Monday night.

From 10 pm on Monday, no people will be allowed in or out of Ruili's urban area for a week except under special circumstances, and all residents will be placed under home-based isolation, Xie Dapeng, the city's mayor, told a news conference.

Those who had left Ruili since Saturday should report to community authorities at their destinations and take nucleic acid tests, he said.

The two imported cases are both Myanmar citizens, one of them a 32-year-old female surnamed Yang and the other a 16-year-old nanny hired by Yang.

They crossed the border from Myanmar to China illegally on Sept 3, along with three of Yang's children and another nanny. They lived together in an apartment in a residential community. On Sunday the pair were confirmed to have contracted the novel coronavirus.

A total of 190 close contacts were tracked down by 4 pm on Monday and 98 of them who had taken the tests showed negative results.

The two-block residential community where they lived has been locked down since 7:30 pm on Saturday, and tests on 1,185 of its residents returned negative results. There are 1,560 residents in total.

The city has also ramped up border controls, locking down all border villages, prohibiting any cross-border movement and heightening precautions against illegal immigration.

A citywide screening campaign targeting illegal immigrants has also been launched to identify hidden risks that could aid the spread of the virus.

The local government will also enhance communication with Myanmar, transmitting information on the condition of the two cases and urging the country to step up border controls.

The China-Myanmar border in Ruili stretches nearly 170 kilometers and residents on both sides have been interacting closely for a long time, according to Yang Bianqiang, vice-mayor of Ruili. The border actually runs through the village of Yinjing.

To prevent the spread of the virus from overseas, the local government has taken a series of steps to crack down on illegal immigrants, including shutting down illegal ports, confiscating boats and conducting regular patrols. However, Yang said the detection of imported cases in the city had exposed weaknesses in its border control work.

He added that the local government will also send protective equipment and financial support to Myanmar if requested and help the country set up more checkpoints in border villages.