A La Auroria (GUA\MGGT) TRACON Controller cleared an air carrier aircraft to ERBOR intersection in an area of high terrain. The FMS attempted to make a right turn but ATC directed the crew a left turn direct which could have put them below the Minimum Off Route Altitude (MORA).

Narrative

After departure; instructed to contact Departure Control was in radar contact. Departure gave clearance after 10;000 FT proceed to Erbor Intersection. At 10;000 FT Captain selected in the FMS to proceed to Erbor intersection; which resulted in a right turn off of the departure profile. At this time we were within 1;000 FT of the grid MORA (Minimum Off Route Altitude); and climbing; with terrain in sight; and began the right turn direct. Controller queried our turn and we acknowledged the right turn. Controller then advised us that we should have turned left and then direct to Erbor. After review; Captain and I agreed that this was not the clearance we were given and the Controller should have been more specific to proceed left turn to La Aurora VOR and then proceed direct to Erbor. My estimation of what happened was a miscommunication and misleading clearance; which could have put us in a conflict with terrain and or traffic. My advice to the Captain and as a reminder to myself; is do not accept any clearance direct unless you can verify you are above grid MORA or established on a segment of the airway.