Many of the dead are still trapped inside their vehicles.
Lilia Marin, a Red Cross spokeswoman, said: "There are 18 dead and 56 missing."
Rescuers used trained dogs and heavy machinery to search through the rubble, but Freddy Roman, a Red Cross spokesman, said hope of finding survivors was diminishing.
Juan Bautista Quesada, 66, was at his house when the quake struck, collapsing the roof.
"The house was swinging like a hammock," Quesada said.
"It wasn't until it was over that I saw there was blood on the floor ... and I saw I had wounds on my head and shoulder from where the roof had fallen on me."
Silvana Lopez, an Argentine tourist, was driving in a rented car with her family when the quake struck.
"Suddenly the road disappeared in front of us and behind us," Lopez said. She took refuge along with several other people in a nearby field where they spent the night.
The National Emergency Committee said helicopters had flown at least 250 people, including tourists, the wounded and the elderly, out of the region, a mountainous area with few access roads, most of which were blocked by landslides.
Roman said about 45 people were still isolated in cut-off areas, and 1,378 people had sought refuge in shelters.