GUATEMALA CITY - The first tropical storm of the 2010 season hit the Pacific coastline of Guatemala and Mexico on Saturday, killing 12 people under landslides and rockfall triggered by torrential rains.

Tropical Storm Agatha's rains caused a landslide in a precarious hillside settlement of Guatemala City that killed four people and left 11 missing, Guatemalan disaster relief spokesman David de Leon said. Most of the city was without electricity at nightfall, complicating search efforts.

Four children were killed by another mudslide in the town of Santa Catarina Pinula about 10 kilometres outside the Guatemalan capital. And in the department of Quetzaltenango, 200 kilometres west of Guatemala City, a boulder loosened by rains crushed a house, killing two children and two adults, de Leon said.

Agatha formed early Saturday in the East Pacific and moved over land in the evening along the Guatemala-Mexico border, said the U.S. National Hurricane Center in Miami.

The centre of the storm was located 30 kilometres east of Tapachula on Saturday night, moving northeast at 16 kph and packing winds of 65 kph.

More than 850 people were evacuated from their homes because of flooding affecting much of central and southern Guatemala.

Before the rains, Guatemala already was contending with heavy eruptions from its Pacaya volcano that have blanketed the capital in ash and destroyed 800 homes. Officials expressed concerns that Agatha's rains could exacerbate the damages.

Though the storm is expected to lose force as it comes ashore overnight, it still could bring rains of 50 centimetres and as much as 75 centimetres in isolated areas of Guatemala.

"The storm will start to weaken and we hope that on Sunday it will be just a tropical depression," said Romero Garcia of Guatemala's Meteorological Institute. "That is not to say that there won't be heavy rains."

The Pacaya volcano, which is just south of the capital, started spewing lava and rocks Thursday afternoon, forcing the closure of Guatemala City's international airport. A TV reporter was killed by a shower of burning rocks.

Airport official Felipe Castaneda told reporters Saturday that the airport would be closed for the next five days while ash is removed.

"The work to remove the ash was going forward, but the rain has complicated it," Castaneda said.

In El Salvador, authorities began evacuating hundreds of families in areas at risk for landslides and flooding, suspending fishing and tourism along the Pacific coast.

Five days of steady rainfall has already swollen a major river flowing through the capital San Salvador.