A U.S. official in Washington said Thursday the embassies would be closed Friday due to "threat information that (was) being taken seriously."
But officials declined to detail what kind of threat was received.
U.S. diplomat Mark Davidson in Asuncion, Paraguay, said the closures were related to a State Department mandate to review security in U.S. embassies in South America.
An official at the U.S. embassy in Quito, Ecuador added: "We need to evaluate the situation and wait to see if we can open again Monday, but the decision will be made this weekend."
Uruguayan Interior Minister Guillermo Stirling denied reports that embassies were closed because of a threat posed by Palestinians who entered Uruguay through Brazil.
"The issue seems to be coming from Ecuador, where an informant said there could be an attack there or in Paraguay or Uruguay, but I don't have any more information than that," said Stirling.
"The embassy asked us to up security around the embassy and the ambassador's home," he added.
In Ecuador, a consul office in Guayaquil was also closed.
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