Cancun, Mexico (CNN) -- A shouting match involving the presidents of Venezuela and Colombia is the latest rumble in months of tension between the two South American nations.

The presidential outbursts between Hugo Chavez of Venezuela and Alvaro Uribe of Colombia happened Monday afternoon at a closed-door luncheon for heads of state attending the so-called "Unity Summit" being held near the Mexican beach resort of Cancun.

According to a source who attended the luncheon and asked not to be named because of the sensitivity of the situation, Chavez and Uribe started yelling and called each other names, using obscene language.

The source also said that Cuban President Raul Castro had to intervene to stop the verbal fight, asking, "How is it possible that we're fighting at a summit intended to unite Latin American and Caribbean countries?"

When asked about the incident, Mexican President Felipe Calderon declined to address the incident, but a spokesman for him confirmed that it happened.

The tension between Chavez and Uribe started last year after the Venezuelan president strongly criticized Uribe for allowing United States forces to be stationed at Colombian military bases.

Tensions have escalated as the countries accused each other of attempts to destabilize each other.

Colombia has accused Chavez's government of aiding leftist guerrilla groups that fight in Colombia. Meanwhile, Chavez accused Colombia of sending unmanned planes to spy on Venezuela.

The shouting match between Chavez and Uribe raises questions about the possibility of creating a regional organization that would speak with a unified voice for Latin American and Caribbean nations, one of the stated goals of the summit.

More than 30 leaders of Latin American and Caribbean nations are attending the two-day summit.