Carter has called for all countries in the Asia-Pacific region to halt the construction of artificial islands.
Tensions are rising over China's rapidly expanding building projects, and Beijing's aggressive stand worries the US and its regional partners.
Carter spoke to reporters during a stop at a Vietnamese Navy base today.
China is vigorously defending the projects in the face of persistent criticism from US leaders.
The Pentagon chief also toured a Coast Guard ship, and tomorrow he will meet with Vietnamese leaders. Other topics expected to come up are maritime weapons sales.