Washington blamed Maduro for the death of Rafael Acosta Arevalo, which came amid a standoff between Venezuela's opposition chief Juan Guaido and the president that's stretched for more than five months.

"The United States condemns the killing and torture of" Acosta, the State Department said in a statement.

The officer "died while in the custody of Maduro's thugs and their Cuban advisers," it added.

National Assembly leader Guaido, recognized as interim president by the United States and about 50 countries, said on Saturday evening that Acosta died "after being tortured."

Acosta was part of a group of 13 people arrested for alleged involvement in a failed "coup d'etat" against Maduro, which the government has tied to Guaido.

Maduro's government has claimed the coup was to have taken place on June 23 and 24 and involved the assassination of the president and several other senior officials.

The Lima Group, made up of a dozen Latin American countries and Canada, condemned the "assassination" of Acosta and called for the intervention of UN High Commissioner for Human Rights Michelle Bachelet.

Bachelet, a former Chilean president, visited Caracas last week and called for the "release" of imprisoned political opponents in the country, which NGO Foro Penal says number close to 800.

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Tamara Suju, a Venezuelan lawyer and human rights activist exiled in the Czech Republic, tweeted that Acosta appeared in court on Friday in a "wheelchair with severe signs of torture."

Venezuela's defense ministry confirmed in a statement that Acosta had "fainted" during the court hearing and that the judge ordered him transferred to a hospital, "where he died."

Without referring to the mistreatment allegations, Attorney General Tarek William Saab, who is close to Maduro, announced an "objective, independent and impartial investigation" following the officer's death.

The National Assembly made a series of demands following Acosta's death, including an investigation by the UN rights chief, an autopsy of Acosta by an "independent international forensic team" and a "verification of the state of health" of military personnel held on accusations of "conspiracy."

But the National Assembly's resolutions and laws are considered null and void by Venezuela's Supreme Court and the Constituent Assembly, both of which are controlled by Maduro loyalists.

The death of Acosta comes amid heightened tensions between Maduro's government and the opposition following two rounds of unsuccessful talks held in Norway.

However Maduro recently said "dialogue will continue," without specifying a schedule or agenda.

On top of the fraught political situation, Venezuela is also grappling with its most-severe economic crisis in recent history, with the country in the grip of power cuts and shortages of basic goods and medicines.

According to the United Nations, more than seven million Venezuelans -- a quarter of the country's population -- need emergency humanitarian aid.