Crowley said: 'The United States, as an exceptional country in the world, has to be seen as practising what we preach.' Photograph: Nicholas Kamm/AFP/Getty Images

The former US state department spokesman who resigned over the treatment of Bradley Manning has said he has no regrets about his comments criticising the manner of the soldier's detention, saying it has undermined the investigation into his role as the alleged source for WikiLeaks.

PJ Crowley resigned this month after calling the Pentagon's treatment of Manning "ridiculous and counterproductive and stupid".

His remarks, made during a speech at MIT, were first reported on a blog by Philippa Thomas, a BBC reporter on sabbatical.

In an interview with the BBC, his first since the resignation, Crowley said he had recently been asked why the US was torturing Manning.

"The United States is doing no such thing, but I understand why the question was asked," Crowley said. "I thought the treatment of Bradley Manning - the fact that he had to sleep naked and stand in a jail cell naked - was counter-productive to our broader effort of appropriately prosecuting someone who has violated his oath of office," he told the Hardtalk programme.

Crowley said he was a believer in "something like strategic narratives", saying: "The United States, as an exceptional country in the world, has to be seen as practising what we preach."

Asked if he had realised the effect his comments would have, Crowley said: "Well, I realised that I was challenging another agency of government. Quite honestly I didn't necessarily think the controversy would go as far as it did.

"But I don't regret saying what I said."

Since June last year Manning has been kept in solitary confinement at a Marine Corps prison near Washington awaiting trial on suspicion of giving classified material to WikiLeaks. Earlier this month it was revealed that he is forced to sleep naked in his cell.

His lawyers said his clothes were taken after he made sarcastic comments about using his underwear to commit suicide. The US authorities confirmed Manning was made to relinquish his boxer shorts for about seven hours due to a "situationally driven" event.

Barack Obama has said he has asked Pentagon officials about aspects of Manning's confinement and been assured that they were appropriate.

Asked about Obama's comments, Crowley said: "Again, I can only offer you my view, which is that it is one thing that actions can be legal and it is another thing that actions can be smart. I do think that the prosecution of Bradley Manning is legitimate and necessary.

"The release of 251,000 cables has damaged US interests around the world and more importantly has put the lives of activists who help us understand what's going on around the world in jeopardy. "But I felt his treatment undermines the credibility of the ongoing investigation and prosecution. I spoke my mind and I haven't changed my view."