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

The US Geological Survey said the new, magnitude 6.1 temblor was centered about 11 miles (18 kilometres) south-southeast of Matias Romero in the state of Oaxaca, which was the region most battered by a magnitude 8.1 quake on September 7.

It was among thousands of aftershocks recorded 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 government of Oaxaca state reported that some homes collapsed. A woman died when a wall of her home fell on her in the town of Asuncion Ixtaltepec, and a man died after a wall fell on him in San Blas Atempa.

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 Xadani. Three hotels and two churches were damaged and a highway bridge collapsed. The Federal Police agency said the bridge already been closed due to damage after the Sept. 7 quake.

Bettina Cruz, a resident of Juchitan, Oaxaca, 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.

Nataniel Hernandez said by phone from Tonala, in the southern state of Chiapas, which was also hit hard by the earlier quake, that it was one of the strongest aftershocks he has felt.

"Since September 7 it has not stopped shaking," Hernandez.

US 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.

"At the moment the greatest damage has been to the Ixtaltepec bridge, which should be rebuilt, and structures with previous damage that collapsed," President Enrique Pena Nieto tweeted. He said government workers were fanning out in Juchitan to provide help to anyone who needs it.

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 magnitude 7.1 temblor that has killed at least 307 across the region. Many residents and visitors fled homes, hotels and businesses, some in tears.

And the Popocatepetl volcano near Mexico City spewed a cloud of vapor with some ash about a mile (2 kilometres) into the air Saturday, but experts said it was not related to the quakes. The 17,797-foot (5,426-metre) volcano has been periodically erupting since 1994.

And late Saturday, Tropical Storm Pilar formed close to Mexico's western Pacific coast. Pilar was expected to remain a tropical storm and brush the coast near the resort of Puerto Vallarta, according to the US National Hurricane Center.

Pilar had winds of 40 mph (65 kph) and was located about 70 miles west-southwest of the port city of Manzanillo. It was moving north at about 5 mph (7 kph).

At Mexico City's Xoco General Hospital, which is treating the largest number of quake victims, workers ordered visitors to evacuate when seismic alarms began to blare Saturday.

That included Syntia Pereda, 43, who was reluctant to leave the bedside of her sleeping boyfriend. Jesus Gonzalez, 49, fell from a third-story balcony of a building where he was working during Tuesday's quake and was awaiting surgery.

But she controlled her emotions, went outside and came back when the trembling was over.

"We are getting used to this," Pereda said. "Every so often we hear the alarm ... you say, well, it is God's will."

Alejandra Castellanos was on the second floor of a hotel in a central neighborhood of Mexico City and ran down the stairs and outside with her husband.

"I was frightened because I thought, not again!" Castellanos said.

Mayor Miguel Angel Mancera said there were no reports of significant new damage in the capital, and rescue efforts related to Tuesday's quake were continuing. He reported that two people died of apparent heart attacks during the new temblor.

At the site of an office building that collapsed Tuesday and where an around-the-clock search for survivors was still ongoing, rescuers briefly evacuated from atop the pile of rubble after the morning quake before returning to work removing cement, tiles and other debris.

As rescue operations stretched into Day 5, residents throughout the capital have held out hope that dozens still missing might be found alive. More than half the dead — 169 — perished in the capital, while another 73 died in the state of Morelos, 45 in Puebla, 13 in Mexico State, six in Guerrero and one in Oaxaca.

Along a 60-foot stretch of a bike lane in Mexico City, families huddled under tarps and donated blankets, awaiting word of loved ones trapped in the four-story-high pile of rubble behind them.

Lidia Albarran, whose niece was buried in the collapse of an office building a block away, heard the alarm and worried that the latest quake could endanger those under the pile of rubble.

"You feel fear. Before, earthquakes did not make me afraid, but now ... thinking about all that could have happened in the building," Albarran said.

In a city still on edge, many residents have spoken of lingering anxiety: imagining the ground is moving when it isn't, hearing a police siren wail and thinking it's a quake alarm, breaking into sobs at unexpected moments.

"There is collective panic. I feel afraid even when a car passes by," said Dulce Bueno, who came Saturday morning with her husband and daughter to the hard-hit Condesa neighborhood. They brought suitcases to collect the belongings of their daughter, who lived in a damaged building beside one that collapsed and who is now moving in with them.

"They have told us it is well constructed, that it's a bunker," Bueno said of her own home. "But if the tremors continue, will it hold up?"

Vicente Aparicio, 76, gazed at the building where he lived in southern Mexico City as his wife listened to an engineer explaining the damage it had suffered. He vowed never to return; his family is fortunate enough to have another apartment to go to and the means to go on with their lives.

"But what about those who do not?" Aparicio wondered.

He added: "How does a city recover from a shock like this?"