Chinese official data have understated foreign direct investment from China to Africa by at least $5 billion, said the South Africa-based Standard Bank in a report released on Wednesday.

"In Africa, evidence suggests that Chinese data may underreport the number of investment projects by as much as two thirds," said Jeremy Stevens, the bank's Beijing-based economist.

In 2012, Chinese firms invested $3.5 billion in Africa, accounting for 7 percent of total inflow to the continent.

The report said that rapid growth makes Chinese investments in Africa specifically difficult to track. And many Chinese State-owned enterprises active in Africa have become independent engines, expanding without necessarily going through the approval processes.

And private firms operating in a wide range of sectors are more difficult to track, it added.

"An average project size of Chinese investments in Africa is usually smaller than $10 million, below the official reporting thresholds," Stevens said.

The bank expects China's FDI in Africa to double by 2015.