A strong new earthquake shook Mexico on Saturday, toppling already damaged homes and a highway bridge and causing new alarm in a country reeling from two even more powerful quakes this month that together have killed more than 400 people.

The U.S. Geological Survey said the new, 6.1-magnitude temblor was centered about 11 miles south-southeast of Matias Romero in the state of Oaxaca, which was the region most battered by an 8.1-magnitude quake Sept. 7.

It was among thousands of aftershocks in the wake of that earlier quake, which was the most powerful to hit Mexico in 32 years and killed at least 96 people.

The Oaxaca state government reported that four people were injured in Juchitan and three in Tlacotepec, but none of their lives were in danger. Another person suffered a broken clavicle in the town of Santa Maria Xadani. Three hotels and two churches were damaged and a highway bridge collapsed. The Federal Police agency said the bridge had already been closed because of damage after the Sept. 7 quake.

The state government said in a statement that homes collapsed, but "no human lives were lost."

Bettina Cruz, a resident of Juchitan, said by phone with her voice still shaking that the new quake felt "horrible."

"Homes that were still standing just fell down," Cruz said. "It's hard. We are all in the streets."

Cruz belongs to a social collective and said that when the shaking began, she was riding in a truck carrying supplies to victims of the earlier quake.

Nathaniel Hernandez said by phone from Tonala, in the southern state of Chiapas, that it was one of the strongest aftershocks he has felt. "Since Sept. 7, it has not stopped shaking," Hernandez said.

U.S. Geological Survey geophysicist Paul Caruso said the new temblor was an aftershock of the 8.1 quake, and after a jolt of that size, even buildings left standing can be more vulnerable.

"So a smaller earthquake can cause the damaged buildings to fail," Caruso said.

Volunteers prepare to continue with the search of survivors, in Mexico City Saturday. (José Méndez/EPA-EFE)

President Enrique Peña Nieto tweeted, "At the moment the greatest damage has been to the Ixtaltepec bridge, which should be rebuilt, and structures with previous damage that collapsed."

Jaime Hernandez, director of the Federal Electrical Commission, said the quake knocked out power to 327,000 homes and businesses in Oaxaca, but service had been restored to 72 percent of customers within a few hours.

Buildings swayed in Mexico City, where nerves are still raw from Tuesday's 7.1-magnitude temblor that killed at least 305 across the region. Many residents and visitors fled homes, hotels and businesses, some in tears.