President Hugo Chavez of Venezuela has threatened to halt oil exports to the US if his country is attacked by Colombia - a close US ally.
The threat comes amid an escalating dispute over allegations that Venezuela is harbouring Colombian rebels.
Mr Chavez broke diplomatic ties with Colombia last week and put his army on high alert.
Venezuela is America's fifth biggest source of imported oil, supplying about a million barrels a day.
Mr Chavez said he had received intelligence that "the possibility of armed aggression against Venezuela from Colombia was higher than it had ever been".
"If there was any attack on Venezuela from Colombian territory or from anywhere else, promoted by the Yankee empire, we would suspend oil shipments to the US, even if we have to eat stones," he said.
"We would not send one more drop to US refineries."
The left-wing Venezuelan leader also said he had cancelled a trip to Cuba to celebrate a revolutionary anniversary with his close ally, President Raul Castro, because of the danger of attack.
A dispute over whether Venezuela allows Colombian Farc and ELN rebels to operate from its territory has dogged ties between the two South American nations for the past eight years.
But relations hit a new low last week when Colombia presented detailed allegations, including maps, photographs and testimony from guerrilla deserters.
Venezuela vehemently denies the accusation, and Mr Chavez has accused Colombia of trying to create a pretext for US military intervention against him.
He has also accused Colombia's outgoing President Alvaro Uribe - who leaves office next month - of trying to prevent an improvement in relations with Venezuela under his successor, Juan Manuel Santos.
Mr Uribe - a close ally of the US - has made the fight against left-wing rebels the main priority of his presidency.
In a newspaper interview published on Sunday, he said he could not understand why guerrilla leaders sheltering in Venezuela were not arrested.
"I leave office with the sadness that these terrorists still have the capacity to inflict damage from outside the country," he said.
In 2008 Mr Uribe sent Colombian troops into neighbouring Ecuador to attack a rebel base, killing the Farc leader, Raul Reyes.
His decision to give US forces access to military bases inside Colombia has been another cause of concern for Venezuela.