BEIJING — A Chinese city bordering Myanmar is stepping up efforts to fight a third coronavirus outbreak after several locally transmitted cases were reported this week.

Ruili, located in the southwestern province of Yunnan, has initiated mass testing and imposed a lockdown to prevent people from entering or leaving the city unless they can prove their travel is necessary, according to the local government.

More than 230,000 test samples have been collected since Monday, and the Jiegao border community was named a medium-risk area on Tuesday, the official Xinhua News Agency reported.

The large number of exchanges along the border at Ruili and Myanmar’s difficulties in handling the pandemic have made it particularly difficult to control new transmissions. Strict anti-pandemic measures have largely prevented local cases in other parts of China over recent months, leaving the country’s death toll from the pandemic static at 4,636 among almost 92,000 reported cases, according to official statistics.