China is starting to dictate terms in one of the world’s strategic waterways, and the United States is largely missing in action.

In his 12-day trip to Asia, U.S. President Donald Trump largely focused on North Korea and trade, all but avoiding the simmering disputes in the South China Sea and steering clear of sharp criticism of Beijing’s increasingly aggressive activities there.

With the Trump administration focused elsewhere for now, China is quietly pressing ahead with its agenda in one of the world’s most strategic waterways, building more military facilities on man-made islands to buttress its expansionist claims and dramatically expanding its presence at sea at the expense of its smaller neighbors.

Beijing’s under-the-radar advances in the South China Sea could be bad news for countries in the region, for U.S. hopes to maintain influence in the Western Pacific, and for the rules-based international order that for decades has promoted peace and prosperity in Asia.

At the Chinese Communist Party congress last month, President Xi Jinping cited island building in the South China Sea as one of his top achievements so far, and touted the “successful prosecution of maritime rights.” Those rights appear at odds with international law: Xi is now assuring nervous neighbors that China will offer “safe passage” through the seas to other countries in the region.

“The South China Sea has fallen victim to a combination of Trump’s narrow focus on North Korea and the administration’s chaotic and snail-paced policymaking process,” said Ely Ratner of the Council on Foreign Relations, who served as an advisor to former Vice President Joe Biden.

China’s recent advances in the South China Sea aren’t as eye-popping as the overnight creation of artificial atolls in recent years, a massive engineering project dubbed the “great wall of sand” by a top U.S. admiral. That’s one reason the disputes got pushed to the back burner on Trump’s big trip.

“Because there’s no sense of immediate or medium-term crisis (in the South China Sea), they didn’t make it a big priority on the trip,” said Evan Medeiros of the Eurasia Group, who oversaw Asia strategy in the Obama White House.

But experts say the quiet moves — including expanding military bases, constructing radar and sensor installations, hardened shelters for missiles, and vast logistical warehouses for fuel, water, and ammunition — are threatening to turn China’s potential stranglehold on the region into reality.

Much of the activity has centered on three reefs converted into artificial islands through large-scale dredging: Fiery Cross, Mischief Reef, and Subi Reef in the Spratly Islands, about 650 miles from Hainan Island in southern China. Satellite imagery in June revealed a large dome had been erected on Fiery Cross with another under construction, suggesting a substantial communications or radar system, experts say. At Mischief Reef, workers were installing two more domes.

With runways, hangars for fighter jets, and communications hardware in place on the artificial islands, China can deploy military aircraft and missiles whenever it wants, solidifying its grip over the area and flouting international maritime law. The three newly built bases in the Spratlys, combined with another on Woody Island, will enable Chinese warplanes to fly over nearly the entire South China Sea, according to Pentagon officials and defense analysts. That could be the precursor to an “air defense identification zone” similar to the one that China slapped onto the East China Sea in 2013.

And the new bases have given China much greater reach at sea. Beijing has deployed more naval ships, Coast Guard vessels, and a flotilla of fishing boats that act as a maritime militia virtually around the clock. The ships can now dock nearby to refuel and resupply, rather than sail home, extending their time on station and their ability to project Chinese power through the area. That is changing the balance of power as fishing ships and coast guard vessels from other claimant countries like Vietnam and the Philippines are elbowed away from disputed features.