MIANYANG, China — The Chinese government warned yesterday that as many 1.2 million residents might have to be evacuated because they could be inundated by a swelling “barrier lake” formed by the May 12 earthquake.

The notice was issued hours after a Russian helicopter transported heavy machines over mountains in the northern part of Sichuan province, and hundreds of Chinese soldiers carried in 10 tons of dynamite, to contend with the barrier lake at Tangjiashan, about two miles upstream from the town of Beichuan.

The afternoon announcement, broadcast on local television, made for another jittery day in Mianyang, a municipality of 5 million people that includes some of the hardest-hit areas of the earthquake, including Beichuan. Hopes of normality returning to the region had been set back by Sunday’s magnitude 6 aftershock, centered north of here, which killed eight people and destroyed 270,000 houses.

The flood warning issued yesterday said that in the worst-case scenario, in which the entire barrier collapsed, about 1.2 million people would be ordered to move to higher ground, including some in central Mianyang. Tens of thousands of people have been evacuated.

The warning prompted some people to haul their tents to higher ground and others to flee Mianyang altogether.

“Some of my friends are leaving town; they want to go as far away as possible,” a taxi driver who drove home after the broadcast to pack up jewelry and other valuables, Liu Decai, 35, said.