US President Donald Trump has made a major intervention into Honduran politics days before the country's presidential election, pardoning a convicted ex-leader and threatening to cut US support if his preferred candidate loses.

Trump said he will pardon ex-president Juan Orlando Hernandez, who was convicted last year in a US court of drug trafficking charges and sentenced to 45 years in prison.

Hernandez, who led the Central American nation from 2014 to 2022, was accused by US prosecutors of facilitating the import of some 400 tonnes of cocaine into the United States.

He was extradited to the United States just weeks after leaving office, when the current president, leftist Xiomara Castro, came to power.

Trump's stunning announcement came in a social media post proclaiming support for Nasry Asfura, the candidate of Hernandez's right-wing party in Honduras's presidential election on Sunday.

The US president had earlier endorsed Asfura, but his latest comments went further, apparently conditioning future aid to Honduras on his victory.

"If he doesn't win, the United States will not be throwing good money after bad, because a wrong Leader can only bring catastrophic results to a country, no matter which country it is," Trump said on his Truth Social platform.

Trump had made a similar threat before Argentina's election last month.

Asfura denied any links to Hernandez but celebrated Trump's endorsement. Hernandez "was president of the Republic, the party is not responsible for his personal actions," Asfura said.

Asfura, a 67-year-old construction magnate and former mayor of the Honduran capital, is running in a tight three-way race against leftist lawyer Rixi Moncada and fellow right-wing TV host Salvador Nasralla.

Trump bashed Moncada, the political heir to Castro, as a "communist" and said her victory would be a win for Venezuela's Nicolas Maduro "and his Narcoterrorists."

The pardoning of Hernandez comes despite a major US operation in Latin America, which Washington says aims to halt drug trafficking, in which over 80 people have been killed in strikes in international waters.

Former US president Joe Biden's attorney general, Merrick Garland, said after Hernandez's sentencing last year that he had "abused his power to support one of the largest and most violent drug trafficking conspiracies in the world."

Trump said in his social media post on Friday that Hernandez "has been, according to many people that I greatly respect, treated very harshly and unfairly," without elaborating. (AFP)