Rescue workers waded through layers of ash - which reached knee-deep in places - only to find charred remains of those who had been unable to flee the molten rock.
"This eruption at Fuego was explosive, sending hot debris down the steep sides of the volcano to make the pyroclastic flows", said Erik W. Klemetti, associate professor of geosciences at Denison University.
Among the fatalities were four people, including a disaster agency official, killed when lava set a house on fire in El Rodeo village, Cabanas said.
Firefighters leave the evacuation area near Volcan de Fuego, or Volcano of Fire, in El Rodeo, Guatemala, Sunday, June 3, 2018.
Seven deaths were confirmed previously. Conred said 3,271 people had been evacuated.
Hilda Lopez said the volcanic mud flowed into her village of San Miguel Los Lotes, close to the mountain.
Rescue operations resumed Monday morning after being suspended overnight due to the unsafe conditions.
Some 3,000 people were evacuated due to the eruption, which affected rural communities around the volcano as well as Antigua Guatemala, a colonial-era town very popular with tourists in the Central American country, he said.
Dozens of videos appeared on social media and Guatemalan television showing the extent of the devastation. People living in villages near the volcano didn't have much time or warning to escape the lava flow, reports CBS News correspondent Don Dahler.
At Least 25 People Killed In Guatemala Volcano Eruption
Garcia said that the El Rodeo village in the southern department of Escuintla is practically buried under lava, adding that the scene is quite delicate as the lava flow hinders authorities from getting into the area.
"We were at a party, celebrating the birth of a baby, when one of the neighbors shouted at us to come out and see the that lava was coming", Lopez remembered.
Volcanic ash covers parked motorcycles, brought by the Fuego Volcano, in Antigua Guatemala.
Mr Morales has declared three days of mourning and toured shelters and the disaster area. Lesser amounts of ash reached the capital of Guatemala City some 25 miles away, forcing the closure of the La Aurora International Airport.
"We saw the lava was pouring through the corn fields and we ran towards a hill", Consuelo Hernandez, a survivor, told Al Jazeera.
Janine Krippner, a volcanologist in West Virginia, noted on Twitter that the Guatemala eruption was unlike the continuing lava flows at Hawaii's Kilauea volcano because it was spewing what scientists call "pyroclastic flows" - quickly moving avalanches that can be "devastating and deadly".
The "Volcan de Fuego", or "volcano of fire", exploded just before noon on Sunday.
Volcanic ash has already stretched across a 12-mile radius, and winds could carry the cloud even farther, officials said.
The eruption officially ended late Sunday, said Guatemala's National Institute of Seismology, Vulcanology, Meteorology and Hydrology.More news: Electronic Arts debuts PC game streaming service at E3 2018
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