As has often been the case in recent years, the worst crisis involves Venezuela's leftist President Hugo Chavez and Colombia's conservative President Alvaro Uribe, with Washington playing a supporting role.
Chavez called on his army earlier this month to "prepare for war" in an angry response to Bogota's decision to give US forces permission to carry out anti-drug operations from Colombian bases.
Colombia for its part has criticised Caracas for racheting up arms purchases from Russia.
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