US Secretary of Defense Ashton Carter voiced deep concerns about the scale of Beijing's island building in the South China Sea.
He made those remarks at the Asia-Pacfic security summit in Singapore.
This comes after the Pentagon revealed that China had put two artillery vehicles on one of the artificial islands it's building.
China says the criticism is baseless, and insists its activities are legitimate.
Several countries have outposts on the region's disputed islands, but China's activities exceed that of the other claimants combined.
That has raised questions about Beijing's motives and there are fears that the dispute could escalate into a deadly conflict.
Presenter: Folly Bah Thibault
Source: Al Jazeera