Paraguay's president has refused to comment on reports that he is the target of an assassination plot.
President Duarte had been on holiday in Brazil
But President Nicanor Duarte Frutos, speaking on Monday after returning from a trip to Brazil, said the US had advised him to reinforce his security.
He could give no further details, Mr Duarte said, but American security experts would work with his own staff.
Local media reported that intelligence services had uncovered a plot by six hired killers to target Mr Duarte.
Mr Duarte held a hastily organised news conference in Asuncion where he told reporters: "The information is classified. I can't go into details."
He said the American ambassador had recently warned him to step up his personal security.
Two Paraguayan newspapers had reported that the president had delayed his return from a holiday in Brazil because of security concerns.
Citing unidentified sources, the newspapers said Paraguay's intelligence service had received reports that a group of men had entered the country armed with anti-aircraft missiles.
Local press reports suggest that the security precautions may have been extended to members of Mr Duarte's family.