At least 43 people including several new born babies have been killed after a bus plunged off a steep cliff in western Guatemala, officials have said.

Rescue workers said the bus, which plunged off of a paved highway about 60km northwest of Guatemala City, was destroyed and more than 40 people were taken to hospitals for treatment.

Investigators do not yet know what caused the crash or what was the final destination of the bus, which was travelling south towards Guatemala City.

"The bus was overloaded," said Sergio Vasquez, a volunteer firefighter at the site.

He said 38 people died at the scene, including six children and 12 women. Five more died at hospitals, he said.

Local television said many passengers were vendors taking farm produce to market.

The government has declared three days of national mourning.

The last major bus accident in Guatemala occurred in 2008 when an overcrowded bus slid off a highway and down a 15-metre slope some 6 km southeast of the capital, killing 53.

Source: Agencies