The first named storm of the 2010 Pacific hurricane season first slammed into the Guatemalan coast near the Mexican border on Saturday.
"Many places are cut off but it appears the weather will improve a bit today and we will be able to airlift supplies to those places. The road network is badly damaged," Alvaro Colom, the president of Guatemala, told a news conference.
At least a dozen people were believed dead in the town of San Antonio Palopo, about 60km southeast of the capital, after a huge mudslide engulfed an entire neighbourhood.
"There was a mudslide that wiped out homes, trees and everything in its path," said a man who gave his name on local radio as Luis.
"We have found 14 bodies and we think there are another eight to 10 beneath the mud."
In El Salvador, nine deaths were reported while in Honduras there were eight deaths reportedly linked to the weather.
Flooding and slides also destroyed 505 homes in Honduras, prompting authorities to evacuate 2,250 people.
Guatemala, El Salvador and Honduras have all declared state of emergencies to facilitate speedy deployment of government aid and resources.
Agatha made landfall near Guatemala's border with Mexico on Saturday with winds up to 75 kph, and was dissipating rapidly on Sunday over the mountains of western Guatemala.
|The storm caused widespread damage in three countries across central America [AFP]
Earlier Colom earlier said the government was considering putting the entire country under a state of emergency, extending one he declared on Friday in an area around the Pacaya volcano.
Hundreds of people are in shelters after the Pacaya erupted on Thursday, billowing clouds of ash that has blocked drainage systems.
The storm-triggered disasters have added to the disruption caused by the erupting volcano,which covered Guatemala City with ash and forced the closure of the capital's international airport.
Agatha also hit El Salvador over the weekend, triggering at least 140 landslides.
The government declared a state of emergency as the Acelhuate river which runs through the capital San Salvador rose to dangerous levels, threatening to flood the city.
Nine people were killed and more than 8,000 were moved to shelters, said Mauricio Funes, El Salvador's president.
In Honduras, where officials have warned of possible mudslides from saturated hills, one man died in the country's north when his home collapsed and a child was swept away by floodwaters.