China's lending to other countries, often shrouded in secrecy, is thought to be higher than the amounts that are officially tracked, resulting in much "hidden debt." That growing debt problem could spark a worse-than-expected slowdown, among other problems, experts warn.

The lack of transparency would also affect investors who are considering bonds issued by those countries, or organizations such as the International Monetary Fund (IMF) which are helping those countries with their debts, according to Carmen Reinhart, a professor at the Kennedy School of Government at Harvard University.

Speaking at the Nomura Investment Forum in Singapore late last month, she said: "China's rise as a global creditor has also meant that there are a lot of hidden debts. That is, countries that had borrowed from China but this borrowing is not reported by the IMF, by the World Bank."

"So there is a tendency to think these countries had lower debt levels than what they actually have," she concluded.

That would hinder the IMF or the World Bank in doing their work on debt sustainability analysis, she said. That effort includes analyzing countries' debt burdens, and coming up with recommendations for a borrowing strategy that limits the risk of debt distress.

"From the vantage point of surveillance, this means that the IMF, if they're doing debt sustainability for example for Pakistan, unless they know how much Pakistan owes China, they are doing that sustainability exercise blindfolded," Reinhart said.

For investors, the limited information they have hinders them in making investment decisions about bonds issued by those countries if they don't know how much is actually owed to China already, she added. That could lead to them underestimating the risk of lending money to those countries through bonds.

Reinhart told the conference that, since 2011, there had been many such loans those countries took from Chinese lenders which needed to be restructured, or renegotiated. Such nations include Sri Lanka, Ukraine, Venezuela, Ecuador, Bangladesh and Cuba, according to Reinhart.

Official debt statistics are tracked by the International Monetary Fund and the World Bank, but that only captures about half the Chinese loans to other countries, Reinhart estimated.

Furthermore, China is not a member of the so-called Paris Club, which also tracks official lending, and is "not interested" in joining, said Reinhart. The Paris Club is a group of creditor nations which aims to fix debt problems of other countries.

Those loans to countries have been shrouded in secrecy, according to reports, with China often demanding public-sector assets as collateral.

One example of those opaque loans is how Chinese loans to Venezuela were denominated in barrels of oil, according to a speech last year from David Malpass, the current president of the World Bank who was then the U.S. Treasury Undersecretary for International Affairs. "This has the effect of masking the exact amount of payments that China made to Venezuelan officials and that Venezuelans are expected to make to China in the future," he said.

"If you ask China for its terms you will not find them," he said in that speech.

Both the IMF and World Bank have called for more transparency on those loan amounts and terms in their annual Spring Meetings in April this year.

In response to a query from CNBC, the World Bank said that debt transparency is "critical."

"Borrowers need comprehensive and timely debt data to make informed decisions. It also allows lenders to manage lending risks more efficiently — thereby bringing down the cost of lending for everyone," it said.

Furthermore, the international organization said, debt transparency lets citizens "hold their governments accountable."

"In short, debt transparency is essential for economic development. So when debts are 'hidden,' that's a problem for everyone — not just the World Bank or the IMF. It's especially a problem for the citizens of countries whose hidden debt is suddenly discovered, since uncertainty can lead to higher funding costs or, in the worst case, cut them off from funding," the World Bank statement said.