McALLEN, Texas (AP) — An agency that monitors the U.S.-Mexico boundary is agreeing to a U.S. proposal to build border fence segments in a South Texas flood plain, a move Mexico opposes.

The decision by the U.S. International Boundary and Water Commission comes despite objections from its counterpart Mexican agency. Mexico argues the fence would deflect floodwaters to its side of the Rio Grande and violate a bi-national treaty.

The Associated Press on Tuesday obtained a letter the commission sent to U.S. Customs and Border Protection noting it will not oppose the project. The commission says its analysis found that the fence proposed for three areas in South Texas would not be a significant obstruction to river waters.

A U.S. Customs and Border Protection spokeswoman says more funding is needed to build the fencing.