When Kim Jin-Sung applied for paternity leave to care for his two young children, the response from senior management at the company he worked for ranged from shock and disbelief to outright anger.

"They asked me hundreds of times if I was being serious," the 40-year-old, Seoul-based IT salesman said.

The reaction from friends was equally incredulous, with most struggling to comprehend why Kim would disrupt his career to do "the wife's job."

But Kim was determined and, after months of pleading with management, he was finally granted a year-long paternity leave -- the first in the 15-year history of his employer.

"It was a very difficult process, but I was really lucky to get it at all," he said in the toy-strewn living room of his apartment in Seoul.

Kim is one of a growing number of South Korea fathers opting to take a break from their careers to help bring up their children -- an unthinkable idea until recently in a male-dominated society where daily childcare has always been considered a woman's responsibility.

But a rock-bottom birthrate, fuelled in part by a growing reluctance among women to accept traditional roles, has spurred the government to push for societal changes that will help couples to have larger families.

The multi-billion campaign has included subsidies to encourage more men to take paternity leave -- despite resistance from the corporate world.

Fathers like Kim forsake their company salary but receive a government handout equivalent to 40 percent of their monthly income -- capped at 1.0 million won ($840). By law men and women are entitled to up to one year maternity or paternity leave.

- Rapidly ageing society -

South Korea has a fertility rate of 1.19 births per woman as of 2013 -- the lowest among OECD member nations and far below the OECD average of 1.67.

This is forcing a worrying demographic shift in Asia's fourth-largest economy, as the pool of young, working-age South Koreans shrinks in contrast to a burgeoning elderly population.

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Sky-high property prices and narrowing job prospects are causing many young South Koreans of both sexes to delay marriage -- or shun it completely.

But women are becoming particularly resistant, rebelling against the traditional norm of the stay-at-home mother, with sole responsibility for all household duties.

South Korea men spend an average of 45 minutes a day on household chores including childcare -- the lowest among OECD member nations.

"Our mindset is still stuck in the old days when men were sole breadwinners," President Park Geun-Hye said last month during a meeting with experts to discuss the low birthrate.

"We need to ... educate more fathers to create a culture where they can naturally take part in childcare," she said, urging local companies to cut "unnecessary long office hours and after-hour drinking sessions."

- Gruelling work culture -

South Korea's intense work culture was a major factor behind Kim's decision to take his year off to be with his five-year-old daughter and three-year-old son.

"I used to come home from work at eight or nine on a few lucky days ... and it was impossible to find enough time to play with or read to my kids," he said.

And he isn't alone.

The number of South Korean fathers taking paternity leave in the first half of 2015 jumped 40 percent from the same 2014 period -- albeit off a very low base -- to 2,212.

But men still only represented five percent of requests for parental leave, compared to more than 40 percent in countries like Sweden.

South Korea still has a "long, long way to go," said Hong Seung-Ah, researcher at Korea Women's Development Institute.

"It's a brave father who asks for a paternity leave in South Korea, but things are changing in a positive way," Hong told AFP.

The "brave" tag, which has been actively promoted by the government, references the concern fathers have of the career consequences.

- Fears of demotion -

A recent survey showed nearly 80 percent of South Korean fathers wanted to take paternity leave, but half were too worried about the risk of being laid off or demoted when they returned to work.

There are signs that the business world is beginning to accept the need for change, even at major conglomerates like Samsung or Hyundai which are notorious for their long office hours and conservative work cultures.

Lee Dong-Hoon, a manager at Hyundai Development Company, decided to become a stay-at-home dad for his baby twins for a year after his wife completed a year-long maternity leave.

Lee's supervisors let him go with little objection, the 38-year-old said -- although he was still the first male employee to take such a long parental break in the firm's 40-year history.

And though many family members and friends doubted the wisdom of his move, Lee said the time he earned with is daughter and son was more than worth it.

"When they learned how to speak, they even said 'dada' before saying 'mama,'" he said, bursting into laughter.

"When they smile at me and say 'dada' in my arms ... a moment like that is just priceless."