By Agence France-Presse
Disaster Relief Agency spokesman David de Leon told a briefing that after hours of searching for survivors, the number of dead had hit at least 65.
In addition, there are 46 people injured, most of them seriously, more than 1.7 million being hit by the disaster, including 3,271 ordered evacuated and 1,787 in shelters in the departments of Escuintla, Sacatepequez and Chimaltenango since Sunday's eruption.
The 3,763-meter (12,346-foot) volcano erupted early Sunday, spewing out towering plumes of ash and a hail of fiery rock fragments with scalding mud.
Authorities had warned the death toll could rise after searches resumed for survivors in communities on the mountain's southern flank.
After an initial toll of 25 dead, it was revised upwards within hours as bodies were recovered from villages razed by the tumbling mud.
"There are missing persons, but we do not know how many," said Sergio Cabanas of Guatemala's disaster management agency. A roll call of communities on the slopes of the volcano was under way.
The speed and ferocity of the eruption took mountain communities by surprise, with many of the dead found in or around their homes.
Cabanas said those who were killed had been overrun by fast-moving burning material discharged by the volcano Sunday. Communities located on its southern slope were the worst hit. Several of the dead were children.
An AFP journalist saw at least three bodies burned in the rubble of the village of San Miguel Los Lotes, where rescue workers, soldiers and police were desperately searching for survivors.
Dead dogs, chickens and ducks also lay among the mud and ash, much of it still smoking.
"I do not want to leave, but go back, and there is nothing I can do to save my family," a weeping Eufemia Garcia, 48, told AFP. She was searching for her three children, her mother, nephews and siblings.
Garcia, from Los Lotes, said she escaped with the help of her husband.
UN Secretary-General Antonio Guterres said he was deeply saddened by the "tragic loss of life and the significant damage caused by the eruption," and said the UN was ready to assist national rescue and relief efforts.
President Jimmy Morales, who has declared three days of national mourning, visited the disaster zone.
"The volcano has erupted before, but never like this," said Gustavo Larius, a 27-year-old bricklayer searching the streets of his village for missing family and friends, a handkerchief pressed over his mouth and nose.
The eruption sent ash billowing over the surrounding area, turning plants and trees gray and blanketing streets, cars and people.
Farmers covered in ash fled for their lives as civil defense workers tried to relocate them to shelters.
"This time we were saved; in another (eruption) no," said Efrain Gonzalez, 52, sitting on the floor of a shelter in the city of Escuintla, where he arrived with his wife and one-year-old daughter.
Gonzalez was overwhelmed with despair, as two more of his children, aged 10 and four, are missing. They were trapped in their home, which was flooded with hot mud that descended from the volcano.
Dense ash blasted out by the volcano shut down Guatemala City's international airport, civil aviation officials said.
The eruption ended after 16.5 hours, but "there is a likelihood that it will reactivate" warned the Institute of Volcanology.
The speed of the eruption took locals by surprise, and could be explained by it producing pyroclastic flows, sudden emissions of gas and rock fragments, rather than lava, said volcanologist David Rothery of Britain's Open University.
"A lava flow rarely travels fast enough to engulf people," he said.
"The videos and still images I've seen suggest instead one or more pyroclastic flows. This is when a violently erupted mass of rock fragments and hot gas finds itself too dense to rise as an ash column and instead cascades down the volcano's slopes.
"Pyroclastic flows or surges can move at over 100 kilometers (62 miles) per hour, and may be hot enough to glow like molten lava. They can travel further, as well as much faster, than lava flows," said Rothery.
Fuego has been erupting since 2002, and was continuously active in 2017. There were explosions and ash plumes and a volcanic mudflow last month.