Venezuela's self-proclaimed acting president Juan Guaido (C) is seen being escorted to his car on February 11, 2020, upon arrival at the international airport near Caracas, after a foreign tour

The US State Department on Saturday condemned the detention in Venezuela of an uncle of opposition leader Juan Guaido, saying the man was being held on "preposterous charges."

"The United States strongly condemns the detention of Juan Jose Marquez, uncle of interim President Juan Guaido, and demands his immediate release," said a statement from State Department spokeswoman Morgan Ortagus.

Marquez was detained Tuesday at the international airport near Caracas while accompanying Guaido on his return trip from a three-week tour to several countries including the US aimed at building pressure on President Nicolas Maduro.

The US is one of more than 50 countries that have recognized Guaido as Venezuela's interim president after Maduro's 2018 re-election was widely denounced as rigged.

The US statement Saturday used uncommonly strong language.

"The preposterous charges that have been put forward further exemplify the increasing desperation of Maduro and his corrupt associates," it said.

Venezuelan authorities claimed that Marquez had smuggled explosive material aboard a flight from Portugal, despite strong denials by Portuguese authorities that he could have done so.

"Manufacturing evidence to justify arbitrary, politically motivated detentions is a common tool of the illegitimate former Maduro regime," the US statement said.

"These despicable actions by the former Maduro regime -- detaining innocent civilians and inventing false charges -- are sadly typical," it said, accusing Maduro and his associates of "mafia-style practices."

Guaido was allowed to leave the airport unhindered Tuesday despite being in defiance of a government-imposed travel ban. He has denounced the treatment of his uncle as "a kidnapping."

The opposition leader said Saturday in Caracas that the detention of his maternal uncle was a "cost" that he is "paying" after returning from his international trip.

"I got the message and here's the answer: We are moving forward and forcefully so. We're not moving an inch on our position," he said, adding that Marquez "isn't a hostage who we will swap."

He had tweeted Wednesday that he holds "usurper" Maduro responsible "for what happens to Juan Jose Marquez, an honest and brave man."

During his three-week tour, Guaido met with the British, French and German leaders and was a guest of President Donald Trump at his State of the Union speech.