Venezuela's President Nicolas Maduro has accused Colombian ex-president Alvaro Uribe of plotting his assassination with sectors of the right wing he says are seeking his overthrow.

Uribe, responding on his Twitter account, dismissed the charges as "immature."

U.S. President Barack Obama would not say Friday whether the United States recognizes Nicolas Maduro as the winner of last month's Venezuelan presidential election.

Obama, in an interview with U.S. Spanish-language network Univision, said the entire region "has been watching the violence, the protests, the crackdowns on the opposition" following the controversial April 14 election.

Venezuela's opposition on Thursday challenged the results of last month's presidential poll won by the late Hugo Chavez's successor, further muddying an already messy transition to life without the divisive leader.

The center-right opposition in the oil-rich South American nation filed its formal challenge with the Supreme Court, even though it says the tribunal is loaded with pro-Chavez judges and certain to reject the challenge.

Venezuelan opposition candidate Henrique Capriles said Wednesday that his campaign will formally contest the official results of his country's presidential elections in a Supreme Court filing.

Capriles told a May Day rally the formal challenge would be presented to the top court on Thursday. President Nicolas Maduro was declared the winner of the April 14 vote to replace the late Hugo Chavez as Venezuela's leader.

Political tensions over Venezuela's disputed presidential election boiled over Tuesday in the National Assembly as government and opposition lawmakers said they physically clashed.

"I'm not the only one who has been beaten. They have struck several lawmakers. (Assembly speaker) Diosdado Cabello has to be held to account personally," said opposition lawmaker Julio Borges.

Venezuelan President Nicolas Maduro met with former Cuban President Fidel Castro on Saturday during a visit to strengthen the "strategic alliance" between the two leftist nations.

The two men spent much of their five-hour visit reminiscing about Maduro's predecessor and mentor, the late President Hugo Chavez, who died last month after a long battle with cancer.

Venezuelan opposition leader Henrique Capriles vowed Thursday to take his challenge of this month's presidential vote to court and repeated his demand for a do-over of the contested election.

Capriles had demanded that Venezuela's National Electoral Board begin reviewing the balloting procedures for the April 14 election he narrowly lost.

The United States voiced hope Wednesday for a "productive" relationship with Venezuela as it welcomed the appointment of a new envoy to Washington by the successor of leftist leader Hugo Chavez.

President Nicolas Maduro on Tuesday named ruling lawmaker Calixto Ortega as the charge d'affaires at the embassy in Washington after the United States clarified that it was not seeking sanctions over Venezuela's disputed election.

Venezuelan President Nicolas Maduro launched a "government of the streets" Monday with new finance and interior ministers, even as his election to replace the late Hugo Chavez remained in dispute.

Other key ministers who had been appointed by Chavez were confirmed in their posts, including the foreign, defense and energy portfolios in a line-up that also ratified Chavez's son-in-law Jorge Arreaza as vice president.

A top election official warned the opposition Saturday against raising "false expectations" over an audit of Venezuela's presidential election, saying it cannot overturn President Nicolas Maduro's win.

Opposition candidate Henrique Capriles accepted the expanded audit in lieu of a vote-by-vote recount on the eve of Maduro's swearing-in Friday, helping to ease tensions that had led to eight deaths in post-election protests.