Thomas Shannon, assistant secretary of state, said on Tuesday that Bolivia's neighbours have "an immediate and direct interest" in having "a successful, stable government" under Morales.
Morales has aligned himself with presidents Fidel Castro of Cuba and Hugo Chavez of Venezuela, and has said that as president he will be the "worst nightmare" of the United States.
A potentially prickly subject in US relations is the production of coca, which, used for thousands of years by indigenous Indians for health and cultural reasons, is also the raw material for cocaine. Bolivia is the world's third-largest cultivator of coca, after Colombia and Peru.
Morales has said he wants to expand the acreage allotted for coca in Bolivia while cracking down on the international cartels that traffic in the plant.
Shannon said increased cultivation, even for local consumption, could provide an opening for the traffickers to expand smuggling operations.
But he said the United States "acknowledges and respects" Morales's solid victory in presidential elections last month.
Shannon discussed Bolivia recently with officials in Brazil and Argentina, which are run by left-of-centre governments that reject extremism. Other neighbours, which Shannon said would have a say about Bolivia's future, are Chile and Peru.
He also mentioned Colombia, which does not share a border with Bolivia.
Condoleezza Rice, the US secretary of state, said last month that Washington "will look to the behaviour of the Bolivian government to determine the course of US-Bolivian relations".
Shannon will represent President Bush at Sunday's inaugural ceremony in La Paz and hopes to meet Morales during the visit.