Along pristine Cambodian beaches, past parades of elephants in its largest national park, sits an area half the size of Singapore that is raising alarm bells among military strategists in the U.S. and beyond.

Dara Sakor, a $3.8 billion China-backed investment zone encompassing 20 percent of Cambodia's coastline, is unlike any other in the developing nation. Controlled by a Chinese company with a 99-year lease, it features phased plans for an international airport, a deep-water seaport and industrial park, along with a luxury resort complete with power stations, water treatment plants and medical facilities.

The size and scope of the plans for Dara Sakor have fanned U.S. concerns that the resort could be part of a larger Chinese plan to base military assets in Cambodia, according to an official familiar with the situation. A naval presence there would further expand China's strategic footprint into Southeast Asia, consolidating its hold over disputed territory in the South China Sea and waterways that carry trillions of dollars of trade.