The first word Mr En learned when he started work on a construction site in Japan after moving from China was "baka" -- "idiot".

The 31-year-old farmer is one of 50,000 Chinese who signed up for a scheme run by the Japanese government that promises to allow foreigners to earn money while they train on the job.

Like many of his compatriots, he hoped to leave Japan with cash in his pocket and a new set of skills that would give him greater chance of getting work at home.

"My Japanese colleagues would always say 'baka' to me," said En, who spoke to AFP on condition that his full name was not revealed. "I am exhausted physically and mentally."

His problem is not the bullying by Japanese colleagues, nor the two-hour each-way commute or the mind-numbing work that largely consists of breaking apart bits of old buildings.

It is the one million yen ($8,700) he borrowed to take part in the programme, apparently to cover travelling expenses and other "fees" charged by middlemen -- which has left him a virtual slave to Japan's labour-hungry construction industry.

"I cannot go back before I make enough money to repay the debt," he said.

Rapidly-ageing Japan is desperately short of workers to pay the taxes to fund pensions and healthcare for its growing grey population, but it is almost constitutionally allergic to immigration.

- Ranks of 'poorly protected' -

Less than two percent of the population is classed as "non-Japanese", the government says. By comparison, around 13 percent of UK residents are foreign born.

The result for Japan, say critics, is ranks of poorly-protected employees brought in through the national back door, ripe for abuse and exploitation.

"This trainee programme is a system of slave labour," says Ippei Torii, director of the Solidarity Network With Migrants Japan, a non-governmental group supporting foreign workers.

"You cannot just quit and leave," he said. "It's a system of human trafficking, forced labour."

Around a quarter of Japan's 127-million population is aged 65 or over, according to recent government figures. This proportion is expected to rise to 40 percent over the coming decades.

The already-heavily indebted government -- which owes creditors more than twice what the economy is worth every year -- is scrabbling to find the money to pay for the burgeoning ranks of elderly, who contribute little in tax but cost a lot in welfare and health.

A far-below-replacement birthrate of around 1.4 children per woman is heaping further pressure on the population.

In most developed nations, this kind of shortfall is plugged by immigration, but Japan allows no unskilled workers into the country, amid fears they would threaten the culture of consensus.

- 'Already here' -

But in 1993 as the economy was on the way down from its bubbly 1980s highs, the government began the Industrial Trainee and Technical Internship Program (TTIP).

The scheme allows tens of thousands of foreigners, mostly from China, Vietnam and Indonesia to come to Japan, supplying labour for industries including textiles, construction, farming and manufacturing.

However, it has been singled out by chief ally the United States, whose State Department's annual Trafficking in Persons Report has for years criticised "deceptive recruitment practices".

"The government did not prosecute or convict forced labour perpetrators despite allegations of labour trafficking in the TTIP," it said in 2014.

Past allegations include unpaid overtime work, karoshi (death due to overwork), and all sorts of harassment, such as a company manager restricting the use of toilets or demanding sexual services.

The Japanese government rejects claims the TTIP is abusive, but acknowledges there have been some upstream problems.

"It is not a system of slave labour," an immigration official told AFP. "It is true that some involved in the system have exploited it, but the government has acted against that."

He said it was not in Japanese authorities' power to control the behaviour of middlemen but insisted such organisations were not allowed to charge deposit fees.

"It is also banned for employers to take away trainees' passports," he added.

Prime Minister Shinzo Abe has unveiled a plan to expand the TTIP to allow workers to stay five years instead of three, and says foreign labour will increasingly be needed, particularly in the booming construction industry ahead of the Tokyo Olympics 2020.

He also knows healthcare must look abroad to plug its shortfall.

"It has been said that we will need one million care-givers for the elderly by 2025, which would be impossible to handle only with the Japanese population," said Tatsumi Kenmochi, a manager at a care home near Tokyo that employs Indonesian nurses.

For her, foreign staff are a precious commodity and the business has to do as much as it can to make them feel welcome.

"It must be hard to leave home and work overseas. We make sure that they don't get homesick, listening to them and sometimes going out to have a warm bowl of noodles, with them."

For Solidarity Network's Torii, this is the kind of attitude Japan needs.

"The issue is not whether we accept immigrants or not," he said. "They are already here, playing a vital role in our society."