The Colombian president has said a letter has been sent to the Union of South American Nations (UNASUR) announcing his country's withdrawal, meaning Bogota will officially cut loose from the bloc in six months.

Newly sworn-in Colombian President Ivan Duque announced on Monday that his country has started a formal withdrawal process stipulated by the organization's constituent treaty, that begins with a member-state sending a written notice to the bloc's headquarters.

"Today with precise instructions, the foreign minister sent UNASUR the letter where we denounce the constituent treaty of that entity and in six months our withdrawal will be effective," Duque announced in a brief televised statement, as he followed through on his pre-election pledge to get Colombia out of the bloc, which was originally intended to foster regional integration and counter US influence when it was founded in 2008.

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Right-wing Duque, who took office on August 7, had previously indicated that Colombia's departure from the 12-member organization was imminent.

Announcing Colombia's withdrawal on Monday, Duque claimed that the bloc of nations had failed to address a political crisis in Venezuela, which has been rocked by a wave of anti-government protests and the attempted assassination of President Nicholas Maduro earlier this year. Echoing the rhetoric of the US White House, Duque has labeled the Maduro government a "dictatorship"; he has also called the UNASUR Maduro's "greatest accomplice."

Colombia became the first and, so far, the only one of the organization's 12 members to formally leave UNASUR. In April, six countries –Argentina, Brazil, Chile, Colombia, Paraguay and Peru– suspended their membership for a period of one year, citing differences over choosing the group's secretary general.

Venezuela, Ecuador as well as Guyana, Suriname and Uruguay continue to remain full members of the bloc.

Prior to Colombia renouncing its membership, Bolivia urged Duque to reconsider his position, hailing the group as "the natural space of integration that constitutes the hallmark of the peoples of the South because we are united by our history and by Mother Earth."