The Ream base is strategically located in the Gulf of Thailand, providing ready access to the fiercely contested South China Sea -- a key global shipping route.
Beijing claims the majority of the resource-rich sea -- vying with six other countries.
"Other countries can also ask for permission to dock ships, re-fuel or hold (joint) exercises with Cambodia," Hun Sen said in a speech at the opening ceremony of a Chinese-owned theme park near the capital Phnom Penh.
Satellite images released by a Washington-based think tank last week showed a US-funded tactical naval headquarters on Cambodia's southern coast had been demolished.
An American embassy spokesman told AFP on Wednesday that the US was "disappointed" by the move, adding the base had been a sign of US-Cambodia relations and was only seven years old.
"We have concerns that razing the facility may be tied to plans for hosting People's Republic of China military assets and personnel at Ream Naval Base."
Such a military presence "would negatively impact the US-Cambodia bilateral relationship and be disruptive and destabilizing to the Indo-Pacific region," the spokesman warned.
The Wall Street Journal last year reported on a secret draft deal allowing China to dock warships at the Ream base.
The Cambodian government in June denied this and maintained its foreign policy was independent.
Cambodia said the facility was simply being relocated about 30km (19 miles) north of Ream as it had outgrown the former site.
Hun Sen has repeatedly insisted Cambodia's constitution forbids any foreign military base within its borders.
In recent years, he has tilted away from the US after Washington's criticism of alleged abuses by his government. - AFP