“If I had arrived 15 minutes later at the hospital, I would have died. I underwent 12 blood transfusions in order to survive.” Sitting in the living room of her wooden stilt house, 39-year-old Buan Kham slowly lifted her skirt to expose what remains of her right leg, amputated at the knee. “If I hadn’t gone to the capital, Vientiane, I would have lost both,” she added, caressing the deep scars running along her left thigh.

Less than a year ago Kham, from the rural village of Na Dee, became one of the 20,000 victims of unexploded ordnance (UXO). The weapons are a lethal legacy of the Vietnam war, which turned this poor, landlocked south-east Asia nation of 6 million into the most bombed country per capita in the world.

It is 50 years since the first US combat troops entered Vietnam in March 1965. During that notorious conflict, the US dropped more than 270 million bombs in Laos as part of a CIA-run, top-secret operation aimed at destroying the North Vietnamese supply routes along the Ho Chi Minh trail and wiping out its local communist allies.

One-third of the bombs failed to explode on impact and have since claimed an average of 500 victims a year, mainly children and farmers forced to work on their contaminated fields to sustain their families. Despite tens of millions of dollars spent, only 1% of Laos territory has been cleared so far.

Last April a Vietnamese metal dealer arrived in Na Dee to buy a 500lb bomb that Kham’s husband had stored under the family house. It had been found a few days earlier in a nearby forest, and he brought it home to sell as scrap metal, a common business that earns local families a few extra dollars. But the bomb exploded when the dealer began to saw off its tail in order to remove some aluminium parts. The dealer died instantly, his face blown away by the explosion. Kham and her two sons, aged 18 and 16, had been sitting nearby. Enveloped in a thick cloud of dust, Kham didn’t realise she had also been hit until she was put on a truck and saw she was covered in blood. She passed out, regaining consciousness 12 hours later at the provincial hospital in Phonsavan – her leg had already been amputated.

Kham spent the next five weeks in another clinic in Vientiane, where she was given a prosthetic leg. She still struggles to get used to it. “It is very heavy, I feel pain every time I use it,” she said. “I am not able to do anything with it and I just spend time at home.”

Her sons were both permanently injured: one lost his left eye; the other damaged his shoulder. As a result, her family are now in dire economic straits. She had to sell all her seven cows to settle the hospital bill and is afraid her husband might abandon her for another woman. “I am worried about my life,” she said in despair. “Both my sons have disabilities. Who will look after the family?”

The legacy of the Secret War, as the American operation is now known, is clearly visible in this idyllic landscape of rolling hills and lush tropical forest. Scarred by thousands of explosion craters, the contaminated area is estimated to be 87,000 sq km , more than one-third of Laos’s territory.

In Xieng Khouang, the most affected province, UXOs are found in forests and school buildings, roads and rice fields. Tim Lardner, the chief technical adviser of UXO Laos – the local company given the task of clearing the country – said: “I have been in this business for 25 years and I have worked in dozens of UXO-affected countries. When I go out on the field, my breath is taken away by the scale of the contamination. It’s like nothing anywhere else.”

According to Kingphet Phimmavong, the company’s provincial coordinator, 85% of UXOs found in Laos were left by the Americans.

For survivors of the war, such as 84-year-old Kampuang Dalaseng, memories are still vivid. “I hate Americans to this date. They bombed, burned and destroyed everything. If their president was here, I would slap him in the face.” A former professor of French, he was forced to flee the bombardments, abandoning the village of Bat Ngot Ngum in 1964 and taking shelter in a forest cave with his family and fellow villagers.

Whenever the American bombers targeted the area the group would move, in an endless cat-and-mouse game. Exhausted and demoralised, the family fled to Vietnam in 1969, embarking on a month-long walk only four days after Dalaseng’s wife had given birth to her second baby. The landscape around them was apocalyptic. “The bombs had destroyed everything,” he remembered. “There were no more animals or villages.”

The most commonly used ordnance by the Americans were cluster bombs, dropped in casings designed to open in mid-air and scatter hundreds of munitions over several hectares. The bombs were designed to explode on impact, lighting up the hills with thousands of simultaneous, deadly explosions. The bombardments were so intense (an average of one every eight minutes, for nine years) that farmers resorted to tending their fields at night. Cooking was severely restricted, because the smoke would attract the aircraft.

Despite their efforts to avoid detection, deaths were commonplace. On 24 November 1968 more than 374 people were killed when an American fighter jet bombed the Tham Piu cave. Today, with its blackened walls and the hundreds of small stone altars erected in memory of the deceased, the place still retains an eerie atmosphere.

UXOs affect not only the daily life of millions of people but the long-term development of the country by delaying the construction of clinics, schools and factories. At the current pace, it will take more than two millennia to clear the country.

Around 40% of the victims are children, who are often attracted by the toy-like shapes of the unexploded cluster bombs. While statistics show risk education programmes carried out by the Laos authorities have contributed to a steep decline in casualties (from 300 in 2008 to 41 in 2013), many accidents in remote areas often go unreported.

In 2014 the Xieng Khouang provincial hospital alone dealt with 14 cases. “The problem will last for long. UXO-injured patients feel the consequences their whole life,” explained 55-year-old Bouanvanh Outhachack, the doctor in charge of the local surgery unit, who has treated more than 500 UXO-related cases in 31 years. Unexploded ordnance claimed the lives of both her grandparents and her mother. “I decided to become a doctor after those accidents,” she said . “Even if I am tired and close to retirement, I still want to help people get back to life.”

Although the US is now the main UXO-clearance donor in Laos, the $82m (£54m) allocated so far is only a small fraction of the $18m per day (inflation-adjusted) that Washington spent to bomb Laos.

From his leafy mansion in Vientiane, US ambassador Daniel A Clune was clearly uncomfortable when speaking about the issue. “We cannot do anything to change the past, we cannot change the history,” he said. “What we can do is to address the current situation.”

In a historic visit in 2012, the then secretary of state Hillary Clinton referred to the war as “the tragic legacies of the past”. The US acknowledged its role in the conflict only indirectly, when it inaugurated a Laos memorial dedicated to the veterans of the Secret War in Arlington National Cemetery in 1997.

Nine years after he fled from Laos, Dalaseng was finally able to return to Bat Ngot Ngum, where he now runs a restaurant with his wife. He is unable to forgive. Like many other Laotians the scars of the war are too deep to heal. “The only way the US can make amends is to clear this country, build roads, schools and hospitals,” he said. “Their bombs are still killing our people every day.”

THE SOUTH-EAST ASIA CONFLICT

1964 The US enters the Vietnam war, saying North Vietnamese patrol boats fired on two US Navy destroyers. US Congress authorises military action in the region. The US begins a nine-year bombing campaign in Laos. The aerial bombardment is an attempt to destroy North Vietnamese sanctuaries and rupture the supply line known as the Ho Chi Minh trail.

1965 200,000 American combat troops arrive in South Vietnam. By 1967 the number has risen to 500,000.

1968 The Tet offensive - a combined assault by Vietcong fighters and the North Vietnamese army on US positions - begins. More than 500 civilians die in the US massacre at My Lai and thousands are killed by communist forces during their occupation of the city of Hué.

1969 North Vietnam’s leader Ho Chi Minh dies. President Nixon begins to reduce US ground troops in Vietnam as public opposition to the war grows.

1973 Ceasefire agreement in Paris. US troops pullout is completed by March.