Venezuelan President Nicolas Maduro accused his Colombian counterpart, Juan Manuel Santos, of "betrayal" on Thursday, saying he had lost faith in him for meeting with Venezuelan opposition leader Henrique Capriles.
Maduro said he was putting bilateral relations, which were frequently hostile during his predecessor Hugo Chavez's 14-year socialist rule, under review following Santos' meeting on Wednesday with Capriles.
The Venezuelan opposition leader, a 40-year-old business-friendly state governor, was in Colombia to press his case that Maduro's presidential election win last month, by just 1.5 percentage points, was fraudulent.
"I'm very sorry President Santos has given credence to the fascist Venezuelan right wing," Maduro, 50, said, again accusing Capriles of plotting to overthrow him and saying the Colombian government was now in league with that plan.
"There is time for rectification, that is what we call for. Meanwhile, we will continue evaluating all our relations with the Colombian government."
Venezuela has already withdrawn its envoy to peace talks in Cuba between Colombia's government and Marxist guerrillas, in protest at the Santos-Capriles meeting.
"I made efforts with the Colombian guerrillas to achieve peace in Colombia," Maduro complained during a speech in the central city of Valencia carried live on state TV. "Now they're going to pay us like this, with betrayal. I've lost confidence in President Santos unless he shows me the contrary."
Colombia is a major U.S. ally and the government before Santos had dire relations with Chavez's administration.
But despite ideological differences, Santos patched things up with Chavez in the name of pragmatism and regional solidarity after taking power in 2010. That helped trade to flow and enabled both sides to chase guerrillas, smugglers and paramilitaries on their long and violence-plagued border.
Still in Bogota on Thursday, Capriles criticized other Venezuelan officials' anger over his meeting with Santos as overblown and unjustified. Minutes after Maduro's comments, he gave a scathing reaction via Twitter.
"He's lost the plot. He doesn't know where he's going - incompetence and illegitimacy combined!"
The refusal of Capriles and his millions of supporters to accept Maduro's election win - which they are disputing in local courts - is infuriating the government, which lambastes him daily as a "fascist" and "coup-plotter."
Authorities are considering legal action against Capriles over post-election protests by the opposition, during which some nine people died. But they are also wary of inflaming the situation given that Venezuelans are split down the middle and arresting Capriles would be bound to spark mass protests.
The election dispute has made for a traumatic start to the post-Chavez era in Venezuela, an OPEC member with the world's largest oil reserves and a population of 29 million.
Chavez died of cancer in March, after naming Maduro as his preferred successor.
The political tensions have also hindered attempts to right an economy that, despite massive oil revenues, is suffering from slowing growth, increasing shortages of basic products, and one of the world's highest inflation rates.