BOGOTÁ, Colombia — The United States extradited a top ally of Venezuela’s authoritarian government on Saturday, his lawyer said, prompting a swift retaliation from Venezuelan officials that immediately threatened a fledgling effort to resolve the country’s political turmoil.

The extradition of Alex Saab, a Colombian businessman and financial fixer for President Nicolás Maduro of Venezuela, to face money laundering charges on American soil was supposed to be a victory for the U.S. government, whose efforts to topple Mr. Maduro have faltered in recent years.

Mr. Saab was detained more than a year ago by law enforcement officials in the West African island nation of Cape Verde. His extradition makes him one of the highest-ranking supporters of Mr. Maduro to be taken into American custody.

But just hours after Mr. Saab was put on a plane to the United States on Saturday, the Venezuelan government re-apprehended six oil executives, including 5 American citizens, who had been under house arrest in Venezuela, according to a lawyer for one of the men.

That same evening, the Venezuelan government also called off negotiations with the American-backed opposition in Venezuela to discuss the conditions necessary to hold free and fair elections.

“By virtue of this very serious action, our delegation announces that it is suspending its participation in the negotiation,” said Jorge Rodríguez, president of Venezuela’s National Assembly and a Maduro ally, in a meeting with reporters on Saturday.

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