A former hitman for Mexican drug kingpin Joaquin "El Chapo" Guzman has offered gruesome testimony in a US court about acts of torture and killings of rivals allegedly committed by the ex-Sinaloa boss.

It was the first time that the federal court heard details of executions carried out by El Chapo himself. The jury had heard other witnesses relate killings ordered by the once powerful drug lord.

While Guzman is not on trial for murder, federal prosecutors want to depict him as a violent and ruthless man ready to do anything to protect his business. He faces trafficking, firearms and money laundering charges that could see him jailed for life in the United States.

The former hitman, Isaias Valdez Rios -- who goes by "Memin" or "Memo" -- told the court on Thursday that he saw Guzman kill a member of the rival Arellano Felix cartel.

The victim had been tortured before he arrived at El Chapo's camp on the plane of Ismael "Mayo" Zambada, who led the Sinaloa organisation with Guzman.

"He had burns made with an iron on his back, his shirt was stuck to his skin. He had burns made with a car lighter all over his body. His feet were burned," said the 39-year-old Valdez Rios.

According to the hitman, who later became Guzman's secretary and pilot, El Chapo became angry when he saw the condition of the man, so he left him locked in a chicken coop for days.

The witness said that after two interrogations, Guzman shot the man, uttering his signature kill phrase: "F*** your mother!" But the kill shot was unsuccessful and the man was still breathing, said Valdez Rios, who worked for seven years in the Mexican army's special forces unit before joining Guzman's organization.

"So we put him in a hole and buried him," Valdez Rios told the court, adding that he had seen El Chapo torture and kill two other members of the Zetas drug cartel.

First, the kingpin beat them nearly to death with a thick tree branch in a wooded area, he said. "They were completely like rag dolls -- their bones were totally broken. They could not move. And Mr Joaquin was still hitting them with the branch and his weapon too," he said.

Near a bonfire that had been lit, Guzman "put the rifle to the head of one guy, fired" and again used his profane kill phrase, the witness said. "Then he did the same to the other guy." The pair were then thrown into the fire, he told the court.

Valdez Rios was arrested in 2014 and has been in a US prison ever since. He is facing a sentence of 10 years to life, but is hoping to see that reduced in exchange for his cooperation with prosecutors.

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