Up to 300 houses have been buried by a landslide in the southern Mexican state of Oaxaca, officials have said.
Heavy rain saturated a 200m (656ft) wide strip of mountainside above the town of Santa Maria Tlahuitoltepec, causing it to slip at 0400 (0900 GMT).
It is not clear how many people have been affected, but the state governor said 500 to 600 could have been buried.
Rescue teams have been delayed because of the bad weather, which has made several roads in the area impassable.
"There has been lots of rain, rivers have overflowed and we're having a hard time reaching the area because there are landslides on the roads," Oaxaca state governor Ulises Ruiz told the Televisa network. He said local telephone lines had also been cut off.
Mr Ruiz added that municipal authorities had told him by satellite phone that the landslide had buried 100 to 300 houses, and speculated that 500 to 600 people could have been asleep inside at the time.
"They're talking about up to 1,000 [people]," he said.
Donato Vargas, an official in Santa Maria Tlahuitoltepec, said the town of 10,000 had been plunged into confusion.
"We were all sleeping and all I heard was a loud noise and when I left the house I saw that the hill had fallen," he told the Associated Press.
Residents were now searching the area for survivors, Mr Vargas added.
The governor said it would not be possible to get a more accurate assessment of the devastation until the first rescuers reached the scene.
"We hope to arrive in time to rescue the families who were buried under the hillside."
The interior ministry said rescue workers from the army, navy and federal police were being flown to the area with rescue dogs and heavy machinery. Specialist crews are also being sent from the capital, Mexico City.
Situated in the Sierra Juarez mountain range, Santa Maria Tlahuitoltepec is famous for its colonial buildings and archaeological sites.
The BBC's Julian Miglierini in Mexico City says the area is the heartland of the indigenous Mixe culture and is considered one of Mexico's poorest.
Two tropical storm systems in the western Caribbean have caused high rainfall in the mountains during the past week.
Parts of Mexico, including Oaxaca, have endured their worst rainy season on record, which has triggered heavy flooding and mudslides which have killed at least 15 people and forced thousands from their homes.