An ex-associate of Joaquin “El Chapo” Guzman has testified that the accused Mexican drug lord once paid a $US100 million ($A139 million) bribe to former Mexican President Enrique Pena Nieto.

Alex Cifuentes, who has described himself as Guzman’s one-time right-hand man, discussed the alleged bribe under cross-examination by Jeffrey Lichtman, one of Guzman’s lawyers, in US federal court.

Asked if he told authorities in 2016 that Guzman arranged the bribe, he answered, “That’s right.”

Pena Nieto has previously denied taking bribes.

The allegations are among the most explosive to emerge from Guzman’s trial, which began in November and has so far featured testimony of lower-level corruption.

Guzman, 61, was extradited to the United States in 2017 to face charges of trafficking cocaine, heroin and other drugs into the country as leader of the Sinaloa Cartel.

Cifuentes testified he had told US prosecutors that Pena Nieto initially reached out to Guzman, asking for $US250 million ($A347.6 million).

Cifuentes told the prosecutors the bribe was paid in October 2012, when Pena Nieto was president-elect, he testified.

He also said testified that Guzman once told him he had received a message from Pena Nieto saying he did not have to live in hiding anymore.

Pena Nieto was president of Mexico from December 2012 until November 2018. He was once a rising star in Mexico’s long-dominant Institutional Revolutionary Party, or PRI.

He was the former governor of the state of Mexico, the country’s most populous, and married a glamorous actress just a couple years before winning the presidency.

But the president ended his term a much diminished figure, pummelled by conflict-of-interest scandals, rampant crime and a lacklustre economy.

Captured by Pena Nieto’s government in February 2014, Guzman broke out of prison for a second time some 17 months later, escaping through a mile-long tunnel dug right into his cell.

The jailbreak humiliated the government and battered the president’s already damaged credibility, though Pena Nieto personally announced news of the kingpin’s third capture when he was again arrested in northwestern Mexico in January 2016.

Colombian-born Cifuentes is one of about a dozen witnesses who have so far testified against Guzman after striking deals with US prosecutors, in a trial that has provided a window into the secretive world of the Sinaloa Cartel, one of the world’s most powerful drug trafficking organisations.