General Joseph Dunford, chairman of the Pentagon's Joint Chiefs of Staff, said he was not calling for military action, but stressed that there was a need to enforce international laws.

"The fall of 2016, President Xi Jinping promised President Obama that they would not militarize the islands. So what we see today are 10,000-foot runways, ammunition storage facilities, routine deployment of missile defense capabilities, aviation capabilities, and so forth," he said in a talk on US security and defense at the Brooking institution.

"So clearly they have walked away from that committment."

"The South China Sea is in my judgement not a pile of rocks," he continued, referring to the series of reefs and outcrops that have been claimed as territory by China, reclaimed and expanded to accomodate military forces and large aircraft.

"What is at stake in the South China Sea and elsewhere where there are territorial claims is the rule of law, international laws, norms and standards."

"When we ignore actions that are not in compliance with international rules, norms and standards, we have just set a new standard."

"I'm not suggesting a military response," the top US general said.

"What needs to happen ... is coherent collective action to those who violate international norms and standards. They need to be held accountable in some way so that future violations are deterred."

Washington has been frustrated by an inability to stall China's aggressive military colonization of the South China Sea, which rejects conflicting territorial claims by five other countries: Vietnam, Malaysia, Brunei, Indonesia and Philippines.

The US has sent navy vessels through the areas claimed by China as "international freedom of navigation operations," but otherwise has found responding difficult.

Dunford acknowledged that building on the Chinese-claimed reefs had slowed.

However, he said, "I assume that's because the islands have now been developed to the point where they provide the military capability that the Chinese required them to have."