"You're dead, it's so hot it sweeps past you, you die", Rothery said. At least 20 others were wounded and still more were missing.
The new eruption spread panic as residents fled areas that had not been under evacuation warnings.
Few of the bodies have yet been identified because the intense heat left them unrecognizable. The volcano, whose name means "fire", had returned to its normal state of activity.
After an initial toll of 25 dead, it was revised upwards within hours as bodies were recovered from villages razed by the tumbling mud.
Sacatepezuez television showed images of the volcano's damage including the charred landscape and three bodies laid partially buried in ash-colored debris from the volcano.
The volcano erupted shortly before noon (4am BST), with lava beginning to flow down from the mountain at around 4pm (11pm BST).
The capital's global airport has also been closed due to the danger the ash poses to planes.
The volcano spewed black smoke and ash into the sky, prompting the evacuation of some 100 people and forcing the capital's La Aurora global airport to shut down its only runway. The airport reopened for private flights on Monday morning.
Israel will coordinate further needs with Guatemalan authorities, he said.
Volcan de Fuego, located near the city of Antigua, is one of the most active volcanoes in Central America.
There is no electricity in the hardest hit areas of Los Lotes and El Rodeo, so most searching continued only until sunset.
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"Conred [the country's disaster agency] never told us to leave".
But by that time, he says, "the lava was already here".
A group of residents arrived at the scene with shovels and work boots.
The conical Volcan de Fuego reaches an altitude of 12,346ft (3,763 meters) above sea level at its peak.
The Volcano of Fire releases lava, seen from Escuintla, Guatemala, Monday, Jan. 4, 2016.
"We will later carry out an evaluation with the Guatemalan authorities", the ministry said in a statement. Palestinians seek East Jerusalem as their future capital and have been infuriated by the embassy moves.
Officials said the casualties occurred mainly in the towns of El Rodeo, Alotenango, and San Miguel los Lotes. Seven deaths were confirmed previously.
Rescuers have struggled to reach rural residents cut off by the eruption, which also wounded at least 20.
One of Central America's most active volcanos erupted in fiery explosions of ash and molten rock Sunday, killing people and injuring many others while a towering cloud of smoke blanketed nearby villages in heavy ash.
Searing flows of ash mixed with water and debris gushed down the volcano's flanks, blocking roads and burning homes.