(Changes headline, adds details on prison security upgrades)
By Lizbeth Diaz and Gabriel Stargardter
MEXICO CITY, Jan 12 (Reuters) - Deeply concerned that the world's most notorious drug kingpin, Joaquin "Chapo" Guzman, could escape for a third time, Mexico has beefed up security at his prison, reinforcing the floor of his cell and placing a guard on his door 24/7.
Guzman, captured on Friday six months after a brazen prison break via a mile-long tunnel that burrowed right up into his cell, is now being held in isolation in another part of the prison, a Mexican security source said.
The improved security measures also include reducing the number of inmates, quadrupling the number of cameras on the site and moving Guzman randomly, without warning, to different parts of the prison, Mexico government spokesman Eduardo Sanchez told Reuters.
"He is being moved randomly from cell to cell ... Since arriving, he's been in eight different cells," said Sanchez.
Juan Pablo Badillo, one of Guzman's lawyers, said his client was physically very weak and complained he was being exposed to "brutal psychological pressure".
"He is in a different, very cold zone (of the prison) and in complete isolation," Badillo told Reuters on Tuesday outside the Altiplano prison where Guzman is being held.
None of Guzman's relatives have visited him yet, Badillo added.
Armored vehicles and light tanks stand guard outside the prison walls, while security forces man sand-bagged checkpoints beyond the gates of the prison, which is about an hour from the capital.
Local media said cells in the prison had been fitted with electronic sensors, additional CCTV cameras and in some cases a mesh of steel bars to reinforce the floor and stymie any efforts to tunnel out.
On Sunday, various senior officials, including National Security Commissioner Renato Sales and Federal Police Chief Enrique Galindo, inspected the improved security measures.
"Today, conditions in the prison fully comply with international standards," the National Security Commission, which runs Mexico's federal jails, said after the four-hour tour.
An interior ministry security official, who has visited the jail on various occasions, said the problem with housing kingpins like "Chapo" was that they could easily buy off the entire prison staff.
In a prison break worthy of Hollywood, Guzman's accomplices apparently obtained the plans to Altiplano and during his 17 months inside built a tunnel fitted with a tiny rail system mounted with a motor bike that he then rode to freedom.
Mexico's government detained around two dozen people in connection with his escape, including the former director of the prison. CCTV video footage showed prison officials watching impassively as Guzman ducked behind a shower wall in his cell and out of sight as he escaped through a hole in the floor.
En route, Guzman disposed of a bracelet that only he and a few other high-risk inmates had to wear, and smashed bulbs lighting up the tunnel as he fled.
In 2001, Guzman bribed guards to help him escape from a prison near the city of Guadalajara after a previous arrest in 1993. He was recaptured in northwestern Mexico in February 2014.
"They are very secure centers. The failures only come via corrupting the personnel," said Jose Alfonso Carreon, who was the deputy director of the high-security jail in Tepic in western Mexico, similar to the one which Guzman escaped from, between 2000 and 2012.
President Enrique Pena Nieto's government says it plans to extradite Guzman to the United States, where he is wanted on an array of charges including drug trafficking, but officials have cautioned the process could take anything from 1-5 years. (Additional reporting by Ana Isabel Martinez; Editing by Simon Gardner and Sandra Maler)