Volcan de Fuego exploded again overnight for the first time since devastating eruptions on Sunday, which officials say have killed at least 75 people and left 192 missing.
National disaster agency Conred ordered evacuations in the town of Escuintla as hot gas and molten rock poured down the southern slopes of the volcano on Tuesday afternoon.
Dr Ralf Gertisser, from Keele University, explained that Fuego had been erupting continuously since at least 2002.
“Fuego is a very active volcano,” he said.
“The population has been used to these previous small eruptions. Everything was largely restricted to the summit of the volcano but the bigger eruption has caught people by surprise.
“The pyroclastic flow will knock down trees, strip houses from basements, toss cars around. It’s highly destructive. It’s the deadliest and most destructive volcanic phenomenon we know.”
Ash billowed more than 5,000m (16,000ft) above sea level, with Guatemala’s seismology and volcanology institute warning that there is a moderate flow of dangerous material down the volcano.
Tuesday’s evacuation orders triggered panic and stalled traffic, even in areas which have not been affected by the latest eruption, with about 2,500 people sheltered in Escuintla after being forced to flee their homes in surrounding villages.
Officials say the search for bodies in mountainous areas destroyed by the eruption has been progressing slowly, with a decision yet to be made on whether it is safe to resume.
Conred spokesman David de Leon said officials were analysing the terrain to make a decision, although the head of the agency has pledged to keep looking for possible survivors.
“We will continue until we find the last victim, though we do not know how many there are. We will probe the area as many times as necessary,” said Sergio Cabanas, head of Conred.
About 10 small eruptions every hour had been reported by seismologists, but none of them compared to the major blast which shook the region on Sunday afternoon.
It unleashed a rapid flow of gas and volcanic debris, which quickly enveloped homes nearby, killing those inside.
Warnings had been issued, but they came too late for some communities to escape, and a new, more powerful explosion followed just two hours later.
Only 23 of the bodies recovered so far have been identified, with many of the victims left unrecognisable.
DNA testing and other methods may be needed to identify them, officials have warned.
Authorities scrambled to issue an evacuation order amidst the chaos and some communities emptied out safely, but in places like Los Lotes and the village of El Rodeo, about eight miles downslope from the crater, it was too late for many.
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Rescuers remain concerned about the possible dangers of further volcanic flows and also rain.
They also expect the number of dead to rise.