Pablo Escobar's most notorious hitman, known by the nickname Popeye, died on Thursday at age 57 after a life of crime he celebrated on YouTube, Colombia's prison authorities said.

Jhon Jairo Velasquez had boasted of killing hundreds of people for his "boss" Escobar, the infamous drug lord killed by Colombian police in 1993 while on the run to avoid extradition to the United States.

Velasquez died at the National Cancer Institute in Bogota, where he'd been receiving treatment for stomach cancer since December 31.

He spent 23 years in prison after giving himself up to law enforcement authorities in 1992 but was arrested again in May 2018 on accusations of "conspiracy to commit crime and extortion," prison authorities said.

Velasquez had been a close associate of Escobar as he ran a drug empire from Medellin that exported thousands of tons of cocaine to the US, making him one of the richest men alive during the 1980s and 90s.

Escobar responded to efforts to extradite him with car bombings, kidnappings and assassinations targeting political leaders, journalists and judges.

Escobar was shot dead by Colombian police on December 2, 1993 in Medellin, his home city.

In an interview with AFP in 2015, Velasquez claimed to have killed "at least 250, maybe 300" people at the behest of Escobar.

Velasquez made a name for himself in prison for his eloquence and the tales he told of his life of crime.

Once out of prison, he launched his own YouTube channel called "Repentant," which had more than one million subscribers.

He was re-arrested in 2018 on the extortion charges, taken into custody at a party hosted by Colombia's head of drug-trafficking investigations.