It's much more dangerous, and twice as expensive, to defect from North Korea since Kim Jong Un took power in Pyongyang three and a half years ago, refugees and experts say, and far fewer people are escaping from the repressive and impoverished country.

With barbed-wire fencing erected on both sides of the Tumen River that marks the border with China, more guard posts and closer monitoring of cross-border phone calls, the number of North Koreans coming annually to the South via China has halved since 2011.

Most defections are arranged through brokers, usually Chinese citizens who are ethnically Korean, and their charges have doubled to about $8,000 per person, beyond the reach of most North Koreans - and that gets them only as far as China.

"Intelligence has stepped up monitoring (of phone calls) on border passages, dampening brokers' activities," said Han Dong-ho, a research fellow at the government-run Korea Institute for National Unification in Seoul, who regularly interviews defectors.

"The more dangerous, the more expensive. Many connections with brokers, which North Koreans call 'lines', have been lost."

The crackdown on defections under Kim has come even as his government has eased restrictions on economic activity, resulting in a slight improvement in livelihoods for many, and providing less reason to escape.

The hundreds of miles of barbed wire strung across T-shaped concrete pillars on the banks of the Tumen were put in place in 2012, according to residents on the Chinese side and historical satellite imagery.

On the North Korean side, guard posts, dogs and shabby concrete watch towers dot the banks of the river, where locals said children from both sides once played together on the winter ice.

"Since Kim Jong Un came in, there have been times where local brokers have refused to go to certain areas on the Chinese side because of the increased security risk," said Sokeel Park of Liberty in North Korea (LiNK), which works with defectors.

There are 27,810 North Koreans resettled in South Korea, according to Seoul's Unification Ministry.

The annual number of defections rose steadily from the late 1990s, according to South Korean government data, when a devastating famine sent desperate North Koreans into China in search of food. It peaked in 2009, when 2,914 North Koreans arrived in the South - the greatest influx since the end of the 1950-53 Korean War.

But in Kim Jong Un's first year in power in 2012, just 1,502 North Koreans made it to the South - a 44 per cent decrease on the previous year. Last year, the number was 1,396.

DISGUISES AND SECRET CODES

From his smoke-filled office in Seoul, human rights activist and defector middleman Kim Yong-hwa manages secret hideouts in China for North Korean refugees, sending them South Korean clothes for disguise and secret codes to communicate with brokers.

"There are still many people who want to cross over to China and to South Korea, but the reality has changed," said Kim, who is himself a defector and heads the NK Refugees Human Rights Association of Korea.

Kim connects North Koreans hiding in China with brokers there, asking the defectors to get new mobile phones or wipe their contacts to keep traceable calls to a minimum.

Tighter border controls, however, have significantly increased the risk - and therefore the cost - of defecting.

Kim, who says he has helped thousands of North Koreans flee the country over the last decade, has considered closing his business this year due to his network of willing brokers dwindling to 20 from about 60 in the past.

"They demand advance payments now, given the risks they have in China," Kim said, adding that he has resources to help only half of the 40 or 50 North Koreans who call him every month.

The overwhelming majority of defectors are female, and come from just two neighboring provinces in the northeast of the country, far from the capital Pyongyang, in an area bordering China where North Koreans considered disloyal under the country's political class system have traditionally been sent.

Unlike men, who tend to have obligations to the state and workplace, North Korean women often have more flexibility and are freer to trade, smuggle, or secretly flee. Women accounted for a record-high 83 per cent of the 292 defections to South Korea in the first three months of 2015.

Those who make the illegal crossing risk being shot, or repatriated and possibly tortured, according to a United Nations report last year.

But beyond the danger of getting caught at the border, an improvement in living conditions in some parts of North Korea may affect anyone's resolve to leave the country. Economically, North Korea has changed since the famine years of the nineties, and a burgeoning market economy means food is easily obtained.

"All things being equal, an improving economy in North Korea, especially in the northeast provinces, would also lead to a decline in defector numbers," said Park of LiNK.

But a gradual improvement in living standards cannot account for the 44 per cent drop in defections under Kim Jong Un, Park said, pointing to the ramped-up border security.

"Compared to 10 years ago the primary motivation for defection has gone from food, to freedom," he said.