Since March, four North Korean teenage girls have not left the back room of Mr. Piao's low brick hovel here, for fear they will be caught and deported. They eat, sleep and pass long boring days crouched on an orange linoleum floor. The only visible sliver of the outside world is a crude ladder outside the back door.

To protect them Mr. Piao, an ethnic Korean Chinese, has placed a fake padlock on the wooden front door and boarded up windows to make his shack appear deserted. But if the police knock, the girls will mount the ladder, scramble over a wall and disappear into the dark safety of the forested mountains.Just a few months ago the teenagers, clad in neat jeans and trendy blouses, were attending computer classes and hairdressing school. 

But that life ended abruptly when 25 North Korean asylum seekers raced through the gate of the Spanish Embassy in Beijing, focusing attention on the plight of North Korean refugees but also prompting a ferocious Chinese crackdown."

Before I could go to the market and hang out with friends," said Kim Gouhua, 18, whose tiny voice and downcast eyes speak to the depression the girls share. "Now my heart is filled with terror."

This year more than 60 North Koreans have gained political asylum in South Korea via China's foreign embassies. But as a result, the hundreds of thousands of North Korean refugees who remain illegally in China have been subjected to previously unheard of levels of scrutiny, detention and deportation.

The measures reflect China's effort to discourage further asylum seekers and placate its ally, North Korea. But they have also shut down a crucial source of food for North Korea's hard-hit northeastern provinces, since migrants in China frequently send food and money home to poor mining areas where state grain distributions ended years ago.

Here in Yanbian, the ethnic Korean region of China that sits along North Korea's border, roadblocks have blossomed this spring like the wildflowers in the surrounding hills, manned by gun-toting border police officers who check vehicles for people who have no papers or cannot speak Chinese.

In cities there have already been several rounds of surprise house-to-house searches. Some ewitnesses say they once saw North Korean soldiers orchestrating brutal roundups of refugees in China.

Of the 30 North Koreans Mr. Piao has sheltered, 7 have been sent back in the current crackdown, double the previous number sent home in the last four years. Aid groups say new arrivals are down almost 50 percent compared with this time last year.

"Fifty-plus have gone to embassies, but 500 have been deported just around here and many more are forced to live like this," said Mr. Piao, pointing to the girls in the corner of his room. "I'd tell anyone who was considering it not to do it."

The United States Congress has implored China not to send the migrants home, with politicians calling on China to grant them access to political asylum. But that solution is at best a pipe dream, given China's friendly relations with North Korea and its fear of being overrun with refugees. In any case, it is not clear how many would qualify as political refugees rather than economic migrants. China has refused to grant United Nations inspectors access to the North Koreans for interviews, so it has been impossible to make that determination.

In the meantime, thousands of North Koreans have taken flight from the border cities that were once their homes to the relative safety of the countryside and the mountains.

A few have found shelter with people like Mr. Piao, a cheerful and deeply committed Christian. He and his wife took in a North Korean girl after their one child, a son, had grown up and left home. One child led to another. But others find a toehold by trading their labor.

Two hours uphill on a rough dirt track crossed by dozens of streams, 14 North Koreans live together in a large crude shack made of mud and planks, sleeping on one large plastic-covered platform. Several hours from Yanji, they harvest corn in exchange for food and a safe haven provided by a prosperous ethnic Korean Chinese farmer.

There are many such farms and small factories all along the border, where North Koreans provide needed cheap labor in one of China's few sparsely populated areas. Families are limited to two children by China's family planning laws, and steep mountain fields preclude farm mechanization. On farms, migrants are paid about $25 a month; Chinese laborers demand twice as much.

Kim Kuinan, a 30-year-old whose skin has turned leathery from his work in the sun, said he and his wife had sought refuge at the farm shack earlier this year after tensions started to rise in the more suburban areas where they had lived since 1996, when they sneaked into China across the frozen Tumen River.

Because he was a coal miner and she a machine operator, they had nothing to eat when the North Korean state grain distributions stopped suddenly in 1995. Mr. Kim tried to survive by selling coal stolen from his mine, but he was soon caught, and once again they were hungry.

Until this year they lived on a large farm at the outskirts of a city, where she tended vegetables and he maintained boilers. But the police knew which houses harbored North Koreans and, when their inspections became increasingly "severe," a local Korean Chinese man helped the couple move to the countryside.

"This farm seems safer than others - it's very remote and the police haven't checked," said Mr. Kim's wife, a small, sinewy woman sitting on the plastic sheet covering the platform that serves as the 14 residents' living room, dining room and communal bed.

But then her husband reminded her that another farm not far down the mountain was raided the week before and 10 North Koreans were arrested. "The events in Beijing have made our lives much more dangerous," he said.

Life is particularly perilous for those who remain in the cities, like the regional capital, Yanji, where some neighborhoods have had three house-to-house searches since March. Here, North Koreans jump at every unexpected noise, sink low at every knock on the door.

One woman who lives in Yanji with her Chinese boyfriend said she had quit her job selling kebabs, leaving her with no money to live on. A tall, fashionably dressed woman in heels and a pink shirt, she blends in readily on the street but is nonetheless terrified of being detected. 

"I used to not worry too much about it, but now I'm really scared to go outside," she said in hushed tones in the upstairs back room of a Korean teahouse.

When she arrived in China, she did not speak a word of Chinese and spent weeks randomly stowing away on trains to places whose names she could not read, until a Korean-Chinese couple helped her. Penniless and ignorant, she ended up sold to - and then escaping from - two different Chinese men. Her price: about $1,200 each time.

In 2000 she was sent back to North Korea after Chinese census takers discovered her at her Chinese boyfriend's apartment. She quickly escaped to China again.

She has spent long periods staring fear in the face, but now she is more rattled than ever. "I wasn't home the first two times they checked our apartment," she said, "but now there is another round going on again."

By Elisabeth Rosenthal, New York Times