Five died at the scene and four others were pronounced dead at San Marcos provincial hospital, volunteer fire-fighter spokesman Oscar Sanchez said.
"That toll could rise, because some had serious injuries," he said.
The accident occurred near the town of Tajumulco, 480 kilometers (300 miles) west of the capital.
Sanchez said the accident occurred in the early morning hours, and that a mechanical problem could have been to blame.

Copyright AFP (Agence France-Presse), 2006