It was love at first sight. From afar, Pham Ngoc Canh, 23, watched the beautiful young woman with the shy smile as she worked in the glass-enclosed laboratory of North Korea's Hungnam fertiliser factory. The North Vietnamese chemistry exchange student thought Ri Yong-hui, a 23-year-old chemical analyst, gracious and sweet. It would be wonderful if she were my wife, he thought – though not a single word had ever passed between them.

It was spring 1971, and while their two countries were allies, both Communist regimes strictly forbade relationships between their citizens and foreigners. Canh knew that even a casual "hello" to Ri was risky. He might be expelled from the University of Chemicals in Hamhung, where he was studying, and sent back to Hanoi. Ri would be severely reprimanded and lose her job.

Still, they exchanged smiles in passing, their eyes meeting in an unspoken connection too powerful to ignore. For the first time in his life, Canh felt himself falling in love, and Ri found herself drawn to the handsome young student with the radiant smile and confident manner.

Canh, who had learnt Korean at the university, discreetly kept track of Ri's work schedule, waiting for a chance to speak to her. He was elated when the two found themselves alone in the lab one day. A light danced in his eyes when he asked: "Have you got a boyfriend?"

Sensing that he was a good man, kind and caring, Ri smiled and said, "No." He asked for her address, and then presented her with a photograph of himself.

Canh returned to his studies just up the road in Hamhung, unable to forget Ri. His heart soared when she replied to his first letter: "Dear Mr Vietnamese Revolutionist Comrade, Thank you very much for your nice gift. I will never forget your beautiful image in the short time we met. I hope your study goes well. I hope that we will have the chance to meet again. Your North Korean friend, Ri. July 20, 1971."

Over the next eight months, they exchanged more letters and met in secret. The North Korean regime rigidly controlled every aspect of its citizens' activities, even their private lives.

Despite the risk, Ri's mother, Kim Trul Sim, and younger sister approved of the romance and helped keep Canh's visits secret. Every few weeks, he boarded a train to see Ri at her small one-room home 15 kilometres away in Hungnam, where they cooked meals together and talked for hours.

Hidden away from prying eyes, their love grew. Although from different cultures, they had much in common. Both lived in countries torn apart by war, and both understood hardship and deprivation. Learning that Ri's father had left for South Korea in the 1950s (something that often stigmatised families left behind) only deepened Canh's desire to take care of her. He showed her photographs of his homeland, dreaming of the day he could take Ri home.

A year after they met, the university sent Canh back to the Hungnam factory for another work exchange. Both acted like strangers at work, but quietly continued to meet in private. The day they dreaded finally arrived: Canh's scholarship was ending and he was expected to return to Hanoi. Their situation seemed hopeless; their governments would never allow them to marry. For Ri, it was worse. Few of North Korea's citizens were ever granted permission to travel within their own country, let alone leave – an act considered a high crime. Distraught, Ri spoke of them dying together.

"I'll wait for you till I die"
On an icy winter day in January 1973, Canh boarded a bus to pay one last visit to Ri. Neither could bear to talk. They sat quietly weeping as the evening shadows lengthened in the tiny house. All too soon it was time for him to leave. Not knowing if he'd ever return, Canh reluctantly turned to Ri's mother. If the opportunity comes, Ri should marry someone else, he said. But in his heart he hoped it would never happen. Canh arrived home just weeks after the massive American bombing of Hanoi and joined in the efforts to re-build the ravaged city. In time, he began work as a chemical designer. Yet no matter what he did, Ri was always on his mind.

Their bittersweet parting was seared in Canh's heart. He re-read the letter Ri had pressed into his hands the day he left, her words bringing him to tears: "If you die in the war, I will die as well." He treasured a photograph of them snapped together, the film developed in secret by a trusted friend. In moments of despair, he too thought that death might be an answer.

Smuggled letters became the fragile lifeline between them. Canh's friends travelling to North Korea posted his letters, while Ri had acquaintances visiting Russia mail hers. Canh secretly saved all of Ri's precious letters, but Ri was forced to burn his. If caught keeping old letters, she would be punished. Ri attempted suicide soon after they were forced apart but, as their time apart lengthened, she resigned herself to the bleakness of her life. Her mother often found her crying in her sleep.

In 1978, Canh returned to North Korea. Hoping to see Ri, he persuaded officials to assign him to a three-month factory work exchange near Ri's work place. Nearly a month went by before he could slip away to see her. Although five years had passed, when Ri opened the door, she still looked beautiful and Canh knew he loved her more than ever.

At the end of that visit, Ri pressed another letter into Canh's hands. "The moment by your side was so short," it read. "If only it was thousands and millions of years. I will dream of you tonight. Will you dream of me too?"

Desperate to see each other, they snatched whatever opportunities they could find to meet. At the end of his three-month stay, Ri risked meeting him at a small restaurant near the guest house where he was staying. Hoping to avoid notice, they walked around the grounds of the compound.

Consumed with sadness, Canh turned to Ri. "How long will you wait for me?" Ri searched his face. As time passed, she had worried he might marry another. "What about you?" she asked. For Canh, there was only one answer: "I will wait for you until I die."

A search begins
The years passed. Neither Canh nor Ri married. But in the late '80s Canh felt a surge of real hope as Vietnam slowly relaxed its rules about marrying foreigners. In 1992 he formed a Vietnamese-North Korean Friendship Association, optimistic that it might pave the way for him to see Ri. Taking a bold risk, he wrote to the North Korean embassy in Hanoi seeking permission to marry Ri. He never received a reply.

That spring Cahn made plans to travel to North Korea as a translator for a tae kwon do team. He bought clothes and gifts for Ri and her family. Then hope turned to despair when, lacking proper travel documents, he was forbidden to travel to Ri's province. It was a crushing blow. Fourteen years had passed since he last looked upon her gentle face; they were both 44 years old. But Canh never lost faith. We belong to each other; she is mine and I am hers, he vowed. I will wait for her until the end of my life.

In 1993, Canh wrote more letters to the North Korean embassy. Pressed to prove their relationship, he handed over photocopies of his treasured letters from Ri. Bluntly told that Ri was married, he refused to believe them, in part because Ri had sent him a letter a year earlier promising to keep waiting for him. Then in July 2001, a clerk at the embassy delivered devastating news. Ri had died ten years earlier. Weeping, Cahn asked for his help in locating Ri's grave so that he might visit her.

Deeply shaken, Canh brooded at home. The clerk had said Ri had died sometime in 1991. Mulling it over, he remembered the last time he had heard from her was in September 1992! Though he had no way to confirm it, he was convinced she was still alive. Refusing to give up, Canh kept sending letters to the North Korean embassy and making inquires. That November, the clerk sent him a note apologising; it was Ri's younger sister who had died.

Learning about a Vietnamese state visit to Pyongyang in May 2002, Canh asked his father for help. His father, a retired diplomat, took letters from Canh to the foreign minister and President Tran Duc Luong, asking them to plead his case with North Korean officials.

A translator friend who accompanied the presidential delegation brought back astonishing news: the two countries had discussed Canh and Ri's relationship. This was the breakthrough Canh had long dreamt about, but he suspected it would be years before anything happened. Then, four months later, Canh returned from a trip to be greeted with the news that not only would the marriage be allowed, but the couple could live where they chose. Afraid North Korea might shut down its borders and rescind Ri's rare exit visa because of escalating tensions with the United States, Canh rushed to bring her home, arriving in Pyongyang on October 6, 2002. Before he was allowed to see Ri, he endured 12 anxious days paying respect to officials and filling in forms seeking permission to take her home.

Lost for words for love
Brought to Pyongyang, Ri stood quietly in a room full of North Korean and Vietnamese officials. She felt as if she were in a dream. She shyly gazed at the man she hadn't been allowed to see in over 24 years, the moment bittersweet beyond words. He looks older now, she thought sadly, pained by the years lost between them.

People mingled around them, but there was no need for Canh or Ri to speak. Their eyes already told the story of a deep, unshakeable love that had survived years of separation. In their hearts, they had been with each other all along.

Married in a civil ceremony in Pyongyang, they later held a joyous wedding celebration with 700 relatives and friends in Hanoi, where they now live. Canh has always felt that their love was a beautiful thing. "And every beautiful thing must be realised one day," he says, proving it through sheer persistence and heroic effort. "Everyone has the right to love. I just tried to have that right."