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"They demand not only more cash, but also all kinds of things for themselves and their superiors," said Mr. Kim, a South Korean human rights activist who helps North Koreans flee their totalitarian homeland and resettle in the South. "They've developed a taste for South Korean goods, too."

Under North Korea's new leader, Kim Jong-un, human rights activists and South Korean officials say, it has become increasingly difficult to smuggle refugees out of the country, contributing to a sharp drop in the number of North Koreans reaching South Korea in the past year.

The number of refugees has never been particularly large, since most North Koreans are so impoverished they find it all but impossible to raise the money to attempt an escape. But the tightening of controls at the Chinese border led to a fall of about 44 percent from the previous year in the number of refugees reaching South Korea in 2012. The total was 1,509, according to South Korean government data.

Despite the relatively small number, the flow of North Koreans defecting to South Korea to escape poverty and oppression has long been a major embarrassment for the North. Lately, the Chinese also appear to have tightened their control at the river border to help protect their client government. "The crackdowns in China and North Korea came in tandem," said Mr. Kim, who manages a network of activists and smugglers from his Caleb Mission church in Cheonan, a city about 60 miles south of Seoul. "It's become more difficult for my people to operate in North Korea and China."

A devastating famine in the 1990s caused many North Koreans to flee, and the number of refugees peaked at 2,917 in 2009. Today, about 24,000 people who escaped from North Korea live in South Korea.

In the last years of his rule, Kim Jong-il, the previous dictator and the current ruler's father, added more checkpoints on the roads to the Chinese border, according to South Korean activists and researchers. North Korea built more barriers along the border and rotated patrols more frequently to discourage corruption.

Under Kim Jong-un, who took over a year ago after his father's death, border controls have tightened further, officials and activists say. The government began to jam the Chinese cellphone signals that activists relied on to coordinate their smuggling operations with collaborators in the North. North Korea also deployed equipment to trace cellphone signals.

"That significantly narrowed the window for cross-border cellphone conversations," said Kim Hee-tae, a leader of the International Network of North Korea Human Rights Activists. His group raises money from churches; until last year it typically arranged for 180 to 190 North Korean refugees annually to escape to the South. But this past year, he said, his organization managed to bring in only about 100 people.

"Even after the bribes are paid, there is no guarantee of success," said Do Hee-youn, head of the Citizens' Coalition for the Human Rights of North Korean Refugees, based in Seoul. "We have recently seen cases where border guards were not punished for having taken bribes when they turned over the refugees." Adding to the difficulty, some of the missionaries and brokers involved in the smuggling were rounded up by the Chinese police.

"It just became impossible to use public transportation in China because these days you cannot buy a train or bus ticket without a proper ID, which the North Koreans don't have," said the Rev. Chun Ki-won, another veteran human rights activist, who runs the Durihana Mission, a Christian group based in Seoul.

But for all the tighter controls imposed by the North Koreans and Chinese, there are still ways of slipping through the cracks.

Landing a border assignment is seen by many North Korean soldiers as a chance to make a fortune by collecting bribes from smugglers. The police in North Korea sometimes protect families with relatives in the South so they can take a cut from cash remittances from the South.

North Koreans have also developed an appetite for outside news and entertainment. "If early defectors fled North Korea for sheer 'survival,' an increasing number of North Koreans reaching South Korea flee for 'a better life' than they had in the North," Kim Soo-am, an expert on North Korean refugees at the Korea Institute for National Unification in Seoul, recently wrote.

A group of 15 North Koreans that the Caleb Mission team in Cheonan had smuggled out in early December included a striking example of one such defector: a 29-year-old woman who yearned to become a television celebrity. "She had watched so many South Korean soap operas that she developed an illusion about life in South Korea," Mr. Kim said, pointing out a particularly well-dressed woman in a photograph of the 15 North Koreans. "When we smuggled her out of North Korea, she was already wearing nothing but South Korean-made clothes."