GUATEMALA CITY (Reuters) - At least 18 people were killed in Guatemala on Saturday, including a dozen on a bus that was buried in a landslide, as heavy rains lashed the Central American nation and southern Mexico.

A dozen people died when the bus they were traveling on was suddenly engulfed by mud around 8 a.m. on the Inter-American highway 50 miles outside of the Guatemalan capital, emergency workers said.

Another six people were killed in separate incidents, Guatemalan President Alvaro Colom told reporters.

Tropical weather systems in both the Pacific and the Gulf of Mexico have triggered torrential rains throughout Guatemala and southern Mexico.

Constant rain in the region over the last week has left hillsides saturated with water and with more precipitation forecast for Sunday officials in both Guatemala and Mexico are taking precautions against landslides.

Heavy flooding in the Mexican Gulf state of Tabasco forced thousands of people from their homes. Authorities in neighboring Chiapas and Oaxaca states, which border Guatemala, and the Gulf state of Veracruz also reported serious flooding.

"The bad weather in the southeast has caused the worst rainy season on record. We are marshaling aid for the affected area," wrote Mexican President Felipe Calderon on his Twitter page.

The state power monopoly opened floodgates on some hydroelectric dams in the region, worsening the flooding in some low-lying areas.

No deaths were blamed on the flooding in Mexico.

Mexico's state oil company Pemex, which has extensive oil and gas operations in Tabasco state and offshore in the Gulf of Mexico, reported no problems with production.