At least 508 people are now known to have died in Guatemala alone since Tropical Storm Stan pounded Central America and southern Mexico.
Many are feared dead after landslides hit towns in Guatemala
Guatemala's losses, said to include many children, bring the overall death toll to at least 610 with El Salvador apparently hit the second-hardest.
Rescuers are struggling to reach hundreds of people still missing in the mudslides and floods.
Officials warn another 1,400 could be dead in two Guatemalan villages.
Firefighters' spokesman Mario Cruz said that he did not believe any survivors would be found in the villagers of Panabaj and Tzanchaj.
"I don't believe there are survivors," he told the AFP news agency.
"Already 36 hours have passed. According to estimates we have, 1,400 people were trapped there."
Rescue efforts in one of the worst-affected areas of Guatemala, around Lake Atitlan, were being hampered by frequent landslides after five days of torrential rain.
The BBC's Mariusa Reyes in Lake Atitlan says the remote Mayan highlands area, popular with tourists, has been transformed into a mud-ridden chaos.
A hillside collapsed on to the lakeside village of Panabaj, swallowing hundreds of homes, officials said.
People in the region say very little help has arrived in their communities and some have not eaten for several days.
"There are no words for this. I have only tears left," teacher Manuel Gonzalez, whose school in Panabaj was destroyed, told Reuters news agency.
"There were only houses here, for as far as you could see... It makes you lose hope. There are no children left, there are no people left."
Guatemala's President Oscar Berger asked Congress to declare a state of emergency shortly before officials confirmed the death toll of 508.
Across the region, an unknown number of people remain trapped in their houses, correspondents say, with some 200,000 people forced to flee their homes.
Stan slammed ashore as a Category One hurricane in southern Mexico on Tuesday.
Despite being downgraded to a tropical depression by the end of the day, it triggered major flooding and landslides in the region.
Mexico's President Vicente Fox has travelled to the southern Chiapas region, where the town of Tapachula was devastated by floods.
He was expected to spend Saturday touring Chiapas and neighbouring Oaxaca to assess the damage. People in Tapachula's shelters have complained of a lack of food, clothing and basic supplies.
At least 17 people have died in Mexico as a result of Stan, which caused at least 30 rivers to burst their banks.
In El Salvador, at least 67 people are known to have died. Officials said nearly 54,000 others had reached 370 shelters throughout the country.