Summary：Two Chinese companies have reached a deal to build a 400-km rail line, a steel plant and a sea port in Cambod...

Two Chinese companies have reached a deal to build a 400-km rail line, a steel plant and a sea port in Cambodia worth a combined $US11.2 billion.

The deal, which was signed on Monday, would be by far the impoverished country's biggest-ever investments.

Chinese firm Cambodia Iron and Steel has contracted the China Railway Group to build the railway to link a steel facility in northern Preah Vihear province to a port at the southern commercial island of Koh Kong.

The rail link and port would cost $9.6 billion and the steel plant $1.6 billion.

The country's main opposition party, the National Rescue Party, has raised concerns about how the project will be rolled out.

"The size of the land is very huge. We are very concerned about forest destruction and environment destruction, and also about the displacement of the people," opposition spokesman Yim Sovann told Radio Australia.

Mr Yim said while the opposition welcomed foreign investment, it must be open to public scrutiny.

"The government should make it public what they want to do and invite all investors around the world to participate in the bidding process. This way we can attract good investors and avoid corruption."

The deal is the latest sign of China expanding its footprint in the frontier economies of a booming South East Asia as the United States vies for influence in the region.

Loans and investment have won China some useful political allies in the 10-member Association of South East Asian Nations (ASEAN), which is set to become an integrated trade community by 2016.

All three projects in Cambodia would start this year and are expected to take up to four years to complete.