China placed a city of four million under lockdown on Tuesday in a bid to stamp out a domestic coronavirus spike, with residents told not to leave home except in emergencies.

The restrictions came as China reported 29 new domestic infections -- including six cases in Lanzhou, the provincial capital of northwestern province Gansu.

Officials in Lanzhou said the "entry and exit of residents" would be strictly controlled and limited to essential supplies or medical treatment.

"All types of residential communities are to implement closed management," authorities said in a statement.

The latest rules come on top of strict stay-at-home orders imposed on tens of thousands in northern China, with access to tourist sites already limited and residents advised not to leave the city unless necessary.

China's latest outbreak has been linked to the contagious Delta variant, with the tally from the latest spread exceeding 100 cases over the past week.

Health officials have warned that more infections may emerge as testing is ramped up in the coming days to fight the outbreak, which has been linked to a group of domestic tourists.