Plans for a Latin American summit in Colombia are in disarray after a diplomatic flap that means Cuba may be barred from taking part.

Cuba's participation in the April 14-15 Summit of the Americas in Cartagena, Colombia, is opposed by the United States. The U.S. position contrasts with increasing diplomatic activity between Cuba and its Latin American neighbors.

Brazil pledged a multibillion-dollar aid package to help Cuba's transition to a market economy and other Central and South American nations are building bridges with Havana as it moves slowly toward economic and trade liberalization.

Cuba is still under U.S. sanctions, the longest such sanctions in history. But trade deals signed by President Raul Castro and his senior aides have thrown new lifelines to the Communist Party government. Many reforms that encourage private enterprise run counter to the Communist Party line but the government plans to introduce more of them in the coming months while carrying on with its ideological rhetoric.

The shifting mood in Latin America was reflected in the diplomatic activity that followed U.S. objections to Cuba's participation. Colombian President Juan Manuel Santos flew to Havana for talks that aimed to explain to the Castro brothers why Cuba couldn't attend and to make sure the rejection didn't cause offense.

Santos met the Castros and then talked with ailing Venezuelan President Hugo Chavez, in Havana for cancer treatment.

There's no reaction yet from Chavez or from Brazilian President Dilma Rousseff, who on an earlier visit to Havana who pledged several billion dollars in aid for Cuba.

Cuba's exclusion from the conference is threatening to overshadow the mid-April talks.

Santos said Cuba wasn't invited because a consensus on its participation couldn't be reached. He thanked the Castro brothers for their tacit agreement not to stir up trouble.

But trouble may still lie ahead, because support for Cuba is growing in Latin American countries outside the left-wing bloc backed by Chavez and other populist leaders.

The conference is an example of shifting sands, analysts said. The Americas summit has traditionally drawn bulk of support from the Organization of American States, which has headquarters in Washington. More recently, the populist bloc of nations grouped in the Venezuela-backed Bolivarian Alliance has sought to influence conference proceedings.

ALBA includes Antigua and Barbuda, Bolivia, Cuba, Dominica, Ecuador, Nicaragua, St. Vincent and the Grenadines and Venezuela. Both Nicaragua and Venezuela have said they want Cuba to attend.

Tensions over the Cuba issue have remained understated because of Santos' diplomatic brinkmanship. Colombia is a close U.S. ally but has cultivated both Cuba and Venezuela, staunch critics of successive U.S. administrations.

Venezuelan aides told Caracas media that Chavez intended to return home after the latest round of cancer treatment and would attend the Cartagena conference.

Chavez, 57, is seeking a third six-year term in Oct. 7 presidential election but has seen his campaign weakened by his illness and the opposition's decision to present a united front with a single candidate, Henrique Capriles Radonski, 39. Capriles won February primaries with more than 62 percent of the vote.