In a notice to Americans in Venezuela, the US embassy in Caracas said on Friday an attack on a US target might occur in a three-day window beginning on Sunday and signalled that it might involve a bombing.
"The US embassy has received information of a possible threat against US interests in Caracas sometime between Sunday, 18 January and Tuesday morning, 20 January," it said.
"US citizens are advised to maintain their security awareness," the embassy said in the notice.
Politically motivated bombings
Although the embassy did not elaborate on the specific nature of the threat, it pointed to advice contained in a section of the State Department's latest travel alert for Venezuela about the potential for politically motivated bombings in the country.
"The risk of encountering explosive devices in Venezuela, particularly in Caracas, appears to be on the increase," the department said in the relevant paragraph of the 15 August advisory.
"These appear to be associated with recent political unrest," it said. "Travellers who encounter a strange parcel or abandoned bag should not attempt to identify or move it, but should immediately notify authorities and stay clear of the area."
The embassy notice provided no further details.
But the reference to politically motivated bombings appeared to indicate the threat might be related to anti-US sentiment in Venezuela fuelled by growing antipathy between Washington and Caracas.
Relations between the United States and Venezuela are near an all-time low over numerous policy differences, including Venezuelan President Hugo Chavez's close friendship with Cuban leader Fidel Castro and harsh criticism of the US-led war in Iraq.
"Travelers who encounter a strange parcel or abandoned bag should not attempt to identify or move it, but should immediately notify authorities and stay clear of the area."
US state department statement
An elected leftist-populist, Chavez, who has been fighting domestic efforts to remove him from office, has repeatedly accused the United States of plotting to oust him in league with the Venezuelan opposition.
Chavez is resisting an opposition-led effort for a referendum to cut short his presidency, claiming that many of the signatures on the petition for a vote on the recall are fake.
On Sunday, Chavez charged that the United States was "doing the groundwork" for his ouster a day after he dismissed US national security adviser Condoleezza Rice as a "true illiterate".