The BBC's Jonathan Head reports on an underground network of mainly Christian activists trying to get some of them out of China, increasingly through South-East Asia, where they can move on to South Korea or the US.
We were taken to an anonymous-looking house in a quiet corner of Vientiane, the sleepy little capital of Laos.
It was bare, with little furniture, but there were seven women there busying themselves cooking a Korean meal.
They were mostly young, in their twenties or early thirties, and seemed light-hearted enough.
But this was their last day of hiding after two weeks in this safe house - the next day they would make the perilous crossing of the Mekong River to Thailand.
Laos is a repressive, communist state, closely tied to its giant northern neighbour. If caught by the Laotian police, they might be sent back to China, and from there to a grim fate in North Korea.
Their minders were all Korean.
There was CK, a dignified, elderly man; Mr Kim, a powerful, muscular man full of energy; and a bespectacled pastor, Peter, who read them passages from the Bible and led hymn-singing sessions.
Christianity had become an important comfort to them.
These activists take huge risks themselves. Two of them have spent years in Chinese jails for their efforts on behalf of the refugees.
Harrowing journey
We had been brought in to see the last stage of the operation because they felt the world needed to be more aware of the terrible plight of these refugees.
Spending that night with the group, while they were still haunted by the fear of being forced back to North Korea, was an emotionally wrenching experience.
Their journey through China had been harrowing enough.
At one point they were abandoned near the border with Laos, and spent two days waiting in the hills in pouring rain.
But their stories of life inside North Korea, and their attempts to escape, were appalling.
Jung Ae, a slightly built woman with a badly damaged leg, described crossing the Tumen river to China in March this year.
The freezing water came up to her neck and nearly swept her away. She had to break through ice on the other side with her bare arms.
She had been driven to escape by exhaustion and hopelessness, she told me.
Her family were suspected of disloyalty, and she had to endure hard labour on state farms to survive, sapping her will to live.
She wept as she described leaving her young son behind with relatives, but said she had no hope of providing for him in North Korea.
Sold into marriage
Jong Sin had left two families behind - two daughters in North Korea, and then a husband and son in China.
Hunger had driven her to try to cross the border eight years ago, but she was caught and imprisoned.
There she was forced to watch the guards hang a young woman by her wrists and slowly beat her to death.
When released, she made a second, successful attempt to escape.
She married a man in China in order to survive, and said he treated her well - many North Korean women are sold into marriages as little more than slaves.
But throughout the seven years she lived with him, she was in constant fear of arrest, finally persuading her of the need to get out of China.
Hanah is just 21. She described chronic stomach pain as a child when her family were reduced to eating dumplings made from tree-bark.
From the age of 15 she tried to escape seven times, her young age apparently saving her from severe punishment when she was caught and sent back.
During these escapes she was separated from her parents, who were also trying to flee.
Almost free
By morning the plans for their escape had changed many times, and the group was extremely tense, holding hands and singing hymns as they waited.
Suddenly, the phone call came and they were ordered to run out of the house without their bags into the pouring rain.
We were unable to follow for fear of jeopardising their escape.
The activists pay substantial sums to Laotian boatmen to get them across the Mekong - the river is constantly patrolled by Laotian police.
All we could do was make the easy, legitimate crossing into Thailand, over the bridge, and wait.
It took several hours but finally CK got the call, his face lighting up, and all the activists jumped up and started hugging each other. They had made it.
They were taken to a hotel in Nong Khai, the Thai border town, and had a day's rest before handing themselves in to the Thai police.
The change in their mood was striking: they bubbled with euphoria and relief, taking a childish delight in the smallest luxuries, dancing around the little cluster of palm trees by an ornamental waterfall in the hotel drive, or bouncing on the armchairs in the lobby.
They were free, or almost free.
Thailand still charges all the North Korean refugees who reach there with illegal entry.
After surrendering themselves, they had to go to court, pay a fine of around US$70 each, and they will be kept in police custody, perhaps for weeks or even months, before being allowed to travel on to the US or South Korea.
It is a stressful experience for people with an engrained fear of all police.
The activists would like to see the Thai authorities improve the process.
But Thailand has never forced any North Korean refugees back to where they came from - an approach the activists desperately hope stays in place despite the growing numbers using this route to escape.
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