Don McCullin's most celebrated image is his portrait of a dazed American soldier, entitled Shellshocked US Marine, Hue, Vietnam. It was taken during the battle for the city of Hue in 1968 and, in its stillness and quiet intensity, says as much about the effects of war on the individual psyche as many of McCullin's more graphic depictions of conflict and carnage. The eyes that stare out beneath the grimy helmet are not staring at the camera lens, but beyond it, into nowhere.

Surprisingly, when I ask McCullin about the photograph, which features in this retrospective of his reportage in Manchester, he grimaces. "It kind of gets on my nerves now," he says, "because it has appeared everywhere. It's like the Eddie Adams shot of the execution of a Vietnamese prisoner."

The acclaimed frontline photojournalist Don McCullin speaks about the horrors of conflict and struggling with 'this terrible name, war photographer'. guardian.co.uk

Now 64, and married for the third time, McCullin lives and works in rural Somerset. These days, he concentrates on landscape photography and has just completed what he says will be his last book, the epic Southern Frontiers: A Journey Across the Roman Empire. "It brings me a kind of peace," he says, "until I hear the local hunters shooting. Gunfire is a prelude to war for me. I feel I'm back there on some godforsaken road passing dying soldiers lying in culverts."

There is a sense when talking to McCullin that he carries a great burden of loss and regret. He has, he says, seen too much in his lifetime and it has left its mark on him. He is recognised as our greatest living war photographer, though he bridles at the term. "Whatever I do, I have this name as a war photographer," he says, ruefully. "I reject the term. It's reductive. I can't be written off just as a war photographer."

This extraordinary exhibition will do nothing to help his cause. Alongside his photographs of conflicts in Cyprus, the Belgian Congo, Vietnam, Cambodia, Lebanon and beyond, many of which have not been seen before, it features contact sheets and magazine spreads from his halcyon days at the Sunday Times magazine in the early 1970s. There is also a wealth of personal material: his passports, his army boots and helmet as well as his many cameras, one of which, a beloved Nikon F, was fractured by a sniper's bullet in a rice field Cambodia in 1970 just as he held it up to his face.

In Vietnam, McCullin lived among the American soldiers, many of whom, he says, thought him mad. "They kept offering me guns for my protection and, to their utter astonishment, I kept refusing. A gun has no place in a ­photographer's kit. You are there as an objective observer." He tells me that many of his contemporaries did carry guns, though. "Dana Stone and Sean Flynn [son of the Hollywood actor, Errol Flynn] were straight out of Easy Rider, riding around on motorcycles carrying pearl-handled pistols. Cowboys, really. I think they did more harm than good to our profession."

Both Stone and Flynn, along with McCullin's friend Gilles Caron, were killed in Cambodia in 1970/71, having been captured by the Khmer Rouge. "They were held in a jungle clearing and then put to death in the most appalling way," he says quietly. Another friend, the great Japanese photographer Kyoichi Sawada, was also captured there but somehow talked his way free. He, too, was killed later. I ask McCullin if he feels like one of the lucky ones. "I guess so. I signed up for a job where you have no guarantees. Why should you? War is war, war means death. If you go and come back, you are lucky."

It was Sawada who photographed McCullin as he lay wounded in a field hospital in Phnom Penh in 1970, having been hit by fragments of a mortar shell. McCullin says he was most afraid, though, when he was captured by Idi Amin's soldiers in Uganda and held prisoner for four days. "They dug pits outside our cell. The sense that something awful was going to happen was constant and almost overwhelming."

Closer to home, McCullin created some of the most memorable images of the early Troubles. During a riot in Derry, he was blinded by CS gas and recovered in a dingy house that reminded him of his working-class upbringing in Finsbury Park in north London. "I was caught between the two sides, with the Provos warning me off one day and the British army chasing me the next." His images from there are often surreal: a well-dressed young man nonchalantly carrying a wrapped parcel past a soldier who is taking aim at rioters; a woman taken aback in her hallway as soldiers in riot gear rush down her street like samurai. "Oh, that was just a gratuitous piece of luck on my part," he says, smiling. "That woman in the doorway, she makes the picture, really."

I ask him if, even in the chaos of conflict, he thought about formal composition. "Always. Somewhere in your head, you think about how the image will appear later. You have to. You want people to see it and be impressed." He singles out his great photograph of an American soldier hurling a grenade. "I was as much a target for the snipers as he was, but I got the shot that I saw in my head."

McCullin famously prints his own photographs. Has he ever developed a print and been shocked by the result. "The albino boy," he says, without hesitation, referring to his heartbreaking image of a starving Biafran child clutching an empty corned beef tin. "The day I came across that boy was a killer day for me. There were 800 dying children in that schoolhouse. The boy is near death. He is trying to support himself. And to see this kind of pathetic photographer appear with a Nikon around his neck …"

He falls silent again for a moment. "Some times it felt like I was carrying pieces of human flesh back home with me, not negatives. It's as if you are carrying the suffering of the people you have photographed."

At the Imperial War Museum, McCullin's vivid and sometimes shocking testimony is war reportage as it used to be. He would not, he says, want to be a young war photographer in Iraq or Afghanistan. "No way. I mean, the idea of agreeing to be embedded? No. It's an absolute tragedy. We spent years photographing dying soldiers in Vietnam and they are not going to have that anymore. I understand that, but you have to bear witness. You cannot just look away."