On Saturday, U.S. Defense Secretary Ash Carter called on Beijing to stop amid fresh tensions.

Earlier this week, a U.S. surveillance plane carrying a CNN crew swooped over such artificial islands, triggering eight warnings from the Chinese navy to back off. U.S. officials are concerned about China's militarization of some of the islands and is trying to create public awareness of it.

Carter reiterated the issue of land reclamation at a conference of diplomats and international experts in Singapore.

Militarization

"The United States is deeply concerned about the pace and scope of land reclamation in the South China Sea, the prospect of further militarization as well as the potential for these activities to increase the risk of miscalculation or conflict among claimant states," Carter said in a speech before diplomats in Singapore.

Anger has simmered over the South China Sea among China and neighboring countries, such as Vietnam, the Philippines, South Korea and Japan, which have argued about existing territorial claims and mineral rights underneath them.

Some of those nations have also built up a few new artificial islands as well.

But of the governments participating in land reclamation, Carter singled out Beijing for particular criticism.

"One country has gone much further and much faster than any other, and that's China. China has reclaimed over 2,000 acres, more than all other claimants combined and more than in the entire history of the region. And China did so in only the last 18 months."

Fear of conflict

Many of China's artificial islands are much closer to other countries' mainland shores than they are to its own.

Carter called for an immediate stop to land reclamation by all nations and a stop to the militarization of the existing ones.

Former CIA Deputy Director Michael Morell told CNN this week that the confrontation could "absolutely" present a risk of the U.S. and China going to war sometime in the future.

The U.S. is considering flying surveillance missions closer than it has before over the islands, as well as sailing U.S. warships within miles of them, as part of the new, more robust U.S. military posture in the area.

It is part of President Barack Obama's strategic and economic "pivot to Asia" policy.

Although he expressed concerns over possible conflicts, Secretary Carter said solutions are limited. "We all know there's no military solution to the South China Sea disputes," he said.

He called for renewed diplomacy.

CNN's Jim Sciutto and Ashley Corum contributed to this report.