Beneath the mud and rock that engulfed the small Guatemalan town of Santa Catarina Pinula last week, search crews have found entire families who died huddled together after they were buried alive.

At least 152 people were killed in Thursday's disaster just outside Guatemala City, government officials said on Monday evening, and emergency services chief Alejandro Maldonado said at least 300 people were still unaccounted for.

The Guatemalan Red Cross made an appeal on Twitter for dry ice at its provisional morgue in the town.

The mud that swallowed the El Cambray II neighbourhood in Santa Catarina Pinula, which lies at the bottom of a deep ravine, is so deep that rescue workers are descending 39 feet (12 meters) through narrow shafts to reach the roofs of homes.

"We've found entire families," said Sergio Cabanas, an official at disaster agency Conred. "We found almost all of them huddled together, which means that they were going to try and evacuate but sadly they didn't have time."

"Some died from the impact, some from asphyxiation and some ... from heart attacks," he added.

Rescue efforts have been hampered by the precarious situation at the site, said Cabanas. There were two smaller landslides on Monday. A nearby river has risen by over 3 feet (one metre), and rescue workers fear for the stability of the hillside where the landslide began.

So far, no survivors have been found at the site, and rescue workers say the chances of finding anyone alive under the 120,000 tonnes of earth that buried the area are close to zero.

But search teams vowed to keep up the search.

"Our determination to continue is firm. We're not going to stop until we finish the job. The objective is that nobody is left buried at the site," Maldonado told a press conference.

Guatemala's government, which is in disarray after former President Otto Perez was forced to resign and was arrested on corruption charges last month, declared three days of mourning for those lost in the landslide.

It has vowed to relocate those still living in risky areas nearby to avoid a repeat disaster.