Still-hot volcanic material and the threat of heavy rains displacing loose deposits were posing a serious risk to the safety of emergency workers, said agency spokesman David de Leon.
To date, 99 people are known to have died in Sunday's major eruption of the volcano, with nearly 200 more still reported as missing.
"We should not underestimate the scale of the disaster," International Red Cross chief Francesco Rocca said after visiting the area.
"Critical emergency needs are still enormous, and affected communities will need sustained and long-term support," Rocca said, adding that for the worst affected families, recovery would take at least a year.
"These people have lost everything -- homes, livelihoods and tragically, loved ones."
The White House said the US was sending emergency aid "to help meet food, water and sanitation needs for the affected population."
An American military plane has already transported some burn victims to the US for specialized treatment, the US Air Force said.
Mexico announced it was sending a team of burn specialists while Chile said it would send equipment to provide an early warning of volcanic eruptions.
A team of Cuban doctors resident in Guatemala were providing support in shelters for the displaced.
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