Demonstrators gesture during a rally over the disputed islands in the South China Sea in Manila. The protest took place Aug. 31 in front of the Chinese Consulate. (Ezra Acayan/Reuters)

China appears to be taking new steps to lay down airfields on two reefs in a disputed area of the South China Sea on the eve of Chinese President Xi Jinping’s Sept. 24 arrival in Washington for a state visit.

Commercial satellite photos taken Tuesday for the Center for Strategic and International Studies show that China is flattening, rolling and putting gravel on an area the size of a military runway on Subi Reef, a once-submerged shoal that Beijing has built up into an area suitable for a military base. The flattened area is about 200 feet wide and nearly 1.4 miles long but is expected to grow and be covered with asphalt, say China experts who have examined the satellite photos.

On Mischief Reef, China has also completed, and started pouring fill into, a retaining wall in a space nearly two miles long — part of a process that is identical to what was done earlier on Subi Reef and Woody Island, where an airfield has been completed, the experts say.

The new construction seems certain to strain the meeting between Xi and President Obama, whose national security adviser, Susan E. Rice, was recently in Beijing. The United States has urged China to stop work in the region, and Beijing said in August that it would halt reclamation. But the satellite photos show that construction continues.

While the Tuesday commercial satellite photos were taken for CSIS, a separate Sept. 3 satellite photo posted on the Diplomat news Web site Thursday evening showed the same developments.

Photos: New satellite imagery of Chinese island-building

“When the Chinese government said it had mainly finished the work, it clearly hadn’t,” said Michael J. Green, a senior vice president at CSIS and former senior director for Asian affairs at the National Security Council under President George W. Bush.

“This is a challenge for the White House,” Green added. “How do they talk about this? Do they say, ‘Don’t militarize these islands,’ knowing that the Chinese will do it anyway? Do they say, ‘Don’t continue construction,’ when it’s obvious that it will continue anyway?”

Green said Chinese officials have told him in private that they intend to militarize the reefs and islands with planes, antiaircraft weapons and naval vessels. He said that would allow the People’s Liberation Army air force to have “overlapping air control over the South China Sea, and not just from one airfield but from three.” He said that “it won’t stop the U.S. policy of asserting freedom of navigation, but it makes it a lot more complicated operation.”

The White House did not comment Friday.

“We note China’s August statement that it has halted its reclamation. At the same time, China has also stated its intent to further construct facilities, including for the purpose of military defense,” said Cmdr. Bill Urban, a Pentagon spokesman. “It’s not clear to us that they’ve stopped, and we will continue to watch that situation very closely.”

But “the only way to ease tensions is to stop unilateral, destabilizing actions,” he added. “China’s stated intentions with its program, and continued construction, will not reduce tensions or lead to a meaningful diplomatic solution.”

Mounting political criticism

China claims sovereignty over the entire South China Sea, while its neighbors — Vietnam, the Philippines, Brunei, Malaysia and Taiwan — have rival claims. The Obama administration has urged China to settle those claims peacefully through the Association of Southeast Asian Nations.

The new construction comes as political criticism in the United States has mounted over Xi’s visit. Several Republican presidential hopefuls have said that the state dinner with Xi should be canceled because of disagreements over the South China Sea, cyberspace hacking, new restrictive laws on nongovernmental organizations operating in China, theft of intellectual property, and continuing disagreements over human rights.

The Senate Armed Services Committee plans to hold hearings Thursday featuring testimony by David Shear, assistant secretary of defense for Asian and Pacific security affairs, and Adm. Harry B. Harris Jr., head of the U.S. Pacific Command.

“If China is building two additional airfields at Subi and Mischief reefs, it demonstrates two things,” Senate Armed Services Committee Chairman John McCain (R-Ariz.) said in a statement. “First, that the reclamation has continued despite Beijing’s claims to the contrary. And second, it shows Beijing’s clear intention to militarize the Spratly Islands with airpower using three different man-made islands.” Several of the disputed reefs are within the Spratly Islands.

“Along with radars and surface-to-air missiles, this will give China the capability to enforce an air defense identification zone, and hold the waters of the South China Sea at risk should it choose to do so,” McCain continued.

The construction also coincides with an unprecedented voyage by five Chinese navy ships through the Bering Sea, close to Alaska’s shores the same week Obama was visiting the state. A U.S. expert on the Chinese military, who spoke on the condition of anonymity to protect working relationships, said the ships were probably on their way back from exercises held with Russian vessels in the Mediterranean.

White House spokesman Josh Earnest said Sept. 3 that “these ships were operating in international waters, and there is no indication that their activities were threatening to the United States in any way.”

‘Great wall of sand’

The Pentagon has taken the lead criticizing China’s activities in the South China and East China seas. Although other countries have occupied atolls in the area, China’s size and ambitions have dwarfed those actions.

Earlier this year, Defense Secretary Ashton B. Carter sharply criticized China’s activities, noting the large amount of commercial shipping in the South China Sea. Construction on the reefs also endangers the area’s marine ecosystem, environmental groups say.

“China is changing the facts on the ground, literally, by essentially building man-made islands on top of coral reefs, rocks and shoals.” Adm. Harris said in a July meeting at the Aspen Security Forum. “I believe that China’s actions to enforce its claims within the South China Sea could have far-reaching consequences for our own security and economy, by disrupting the international rules and norms that have supported the global community for decades.”

In March, Harris said that “China is creating a great wall of sand” and warned that its “pattern of provocative actions” was “inconsistent with international law.” He said, “It’s no surprise that the scope and pace of building man-made islands raise serious questions about Chinese intentions.”

Harris and other Pentagon officials have said the United States should conduct freedom-of­-navigation operations by flying or sailing within the 12-nautical-mile limit that constitutes a state’s territorial sea. That limit does not apply to Subi Reef, they say, because the reef is not only disputed but was also submerged and therefore not territory at all.

But they are at odds with the White House, which is said to oppose any such measure now.

“We cannot continue to restrict our Navy from operating within 12 nautical miles of China’s reclaimed features,” McCain said. “This sets a dangerous precedent that grants de facto recognition of China’s man-made claims.”“What does the administration do when Xi Jinping comes to town?” asked Green of CSIS. “I think it’s unlikely that the Chinese side will back down and stop construction.”

Missy Ryan contributed to this report.