At least 1,419 people are known to have died in the 7.2-magnitude quake and an unknown number are still missing.
Tropical Depression Grace is expected to pass over the worst affected area later on Monday.
It is feared heavy rain brought by Grace could trigger landslides.
Roads already made impassable by the quake could be further damaged by the rains, so aid teams are racing to get essential provisions to the quake-hit region before Grace hits.