Unreported Chinese local government debt may amount to trillions of U.S. dollars, meaning the country's debt-to-GDP ratio has hit "alarming" levels, S&P Global Ratings said in a report released Tuesday.

The analysts noted a large gap between reported investment in local infrastructure and funding, as permitted by central authorities. As a result, the actual level of off-balance sheet debt could be several times more than what is publicly disclosed and range as high as 30 trillion yuan to 40 trillion yuan, or about $4.34 trillion to $5.78 trillion, credit analysts Gloria Lu, Laura Li and their team said in the report.

"And that's a debt iceberg with titanic credit risks," they added, estimating that the ratio of all government debt to GDP was 60 percent last year.

To encourage economic growth in the region, local governments in China have invested heavily in infrastructure, often using financing structures known as "local government financing vehicles," or LGFVs. Details about their size or nature tend to be unclear, and the S&P analysts said much of the hidden debt is in those vehicles.

Beijing has been trying to move financing away from off-balance sheet sources, but has had limited success so far. In the future, S&P Global Ratings expects authorities will allow more defaults in local government financing vehicles, the report said.

In September, the ratings agency lowered its long-term issuer credit ratings on seven vehicles, including two in the city of Tianjin, just outside of Beijing.