The US defence secretary has accused Cuba and Venezuela of fomenting unrest in Bolivia, which has led to the overthrow of two presidents since 2003.
Donald Rumsfeld may find it hard to get regional backing
Speaking in Paraguay, a close US ally, he asked South American nations to take a multi-lateral approach to the issue.
But while Donald Rumsfeld has had a warm reception in Paraguay, he will have a hard time persuading other South American leaders, correspondents say.
Many countries in the region are improving their ties with Venezuela.
US Defence Secretary Donald Rumsfeld said there was evidence that both Cuba and Venezuela had been "involved in the situation in Bolivia in unhelpful ways".
He did not give more details.
BBC South America correspondent Tom Gibb says Washington is clearly worried that the strongly anti-US coca grower, Evo Morales, who has led many of the Bolivian protests, could win elections there in December.
Mr Rumsfeld's accusation represents a significant stepping-up of attempts to isolate the left-wing Venezuelan President, Hugo Chavez, our correspondent says.
But countries like Brazil and Argentina are increasing economic and political co-operation with Mr Chavez rather than trying to isolate him.
And many analysts dispute Washington's view, saying the turmoil in Bolivia owes more to home-grown factors than external influence.
Mr Rumsfeld is also due to visit Peru which, like Paraguay, shares a border with Bolivia.