The Republican US presidential candidate John McCain was not tortured during his captivity in North Vietnam, the chief prison guard of the jail in which he was held has claimed.

In an interview with the Italian daily Corriere della Sera, Nguyen Tien Tran acknowledged that conditions in the prison were "tough, though not inhuman". But, he added: "We never tortured McCain. On the contrary, we saved his life, curing him with extremely valuable medicines that at times were not available to our own wounded."

McCain, who fell into enemy hands after his plane was shot down in 1967, has frequently referred to being tortured and has cited his experiences as a reason for vigorously opposing the endorsement by the Bush administration of the use of techniques such as "water-boarding" on terrorist suspects.

Shortly after his release in 1973 McCain told US News & World Report that his prison guards had beaten him "from pillar to post". After being worked over at intervals for four days, he said, he had become suicidal and agreed to sign a "confession" admitting to war crimes.

In his 1999 autobiography, Faith of My Fathers, he described how after his capture he was subjected to inhuman treatment in an effort to force him to disclose his ship's name, squadron number and the target of his final mission. He was threatened with the withdrawal of medical assistance and, while still suffering from his crash injuries, his guards "knocked me around a little".

For his service in Vietnam and his actions as a POW, McCain was awarded the Silver Star, the Legion of Merit, the Bronze Star, the Navy Commendation Medal and the Purple Heart.

Tran, now 75, said McCain reached Hanoi with the worst injuries he had seen in a downed pilot. But he denied torturing him, saying it was his mission to ensure that McCain survived. As the son of the US naval commander in Vietnam, he offered a potential valuable propaganda weapon.

However, recommending McCain for a medal after the war, his former cellmate, the much-decorated Colonel George Day, said the admiral's son had forced his interrogators to "drug him and torture him to get any cooperation", according to a letter in the US National Archives cited earlier this year by the Washington Post. Day said McCain suffered "torturous abuse".

Tran told Corriere that McCain was sent to hospital the day after he was brought to Hanoi and stayed there for a month. "I never lost him from sight. I was frightened a doctor or nurse might do him harm."

Tran dismissed as "absolutely impossible" perhaps the most famous story from McCain's autobiography: that one Christmas, a guard traced a cross in the mud in front of him. "My men were all communists and atheists," he said.

As to why McCain, then 36, left North Vietnam with prematurely grey hair, Tran denied it was because of mistreatment. "It's that in prison you think too much."