Italian Premier Giuseppe Conte meets Chinese President Xi Jinping to sign trade agreements on Belt and Road Initiative, on March 23, 2019 in Rome, Italy.

China's lending to other countries has surged in the past decade, causing debt levels to jump dramatically, and as much as half of such debt to developing economies is "hidden," a new study has found.

Such "hidden" debt means that the borrowing isn't reported to or recorded by official institutions such as the International Monetary Fund (IMF), the World Bank, or the Paris Club — a group of creditor nations.

Between 2000 and 2017, other countries' debt owed to China soared ten-fold, from less than $500 billion to more than $5 trillion — or from 1% of global economic output to more than 5%, according to the study from Germany-based think tank the Kiel Institute for the World Economy.

"This has transformed China into the largest official creditor, easily surpassing the IMF or the World Bank," the report's researchers said.

The study, which looked at nearly 2,000 Chinese loans to 152 countries from 1949 to 2017, was undertaken by well-known debt expert Carmen Reinhart from Harvard University, as well as Kiel Institute's Christoph Trebesch and Sebastian Horn.

For 50 developing countries which have borrowed from China, that debt has increased on average from less than 1% of their GDP in 2015, to more than 15% in 2017, according to estimates by the study's researchers.

"Advanced and higher middle income countries tend to receive portfolio debt flows, via sovereign bond purchases of the People's Bank of China," the report said. "As a result, many advanced countries have become highly indebted towards the Chinese government."

It further added: "Lower income developing economies mostly receive direct loans from China's state owned banks, often at market rates and backed by collateral such as oil."

Much of the lending is done through two policy banks — the China Development Bank and Export-Import Bank of China.

Estimates suggest that China now accounts for a quarter of total bank lending to emerging markets, according to the study.

"China's international lending boom is primarily a result of the country's rapid economic growth, but also due to the "going global" policy of the Chinese state," says Trebesch, who is head of the research area international finance and global governance at the Kiel Institute.

China has been criticized for saddling many countries with debt through its Belt and Road Initiative — a mammoth global infrastructure investment plan to build rail, road, sea and other routes stretching from China to Central Asia, Africa and Europe.