American officials had been waiting to see what China would do around Scarborough Shoal after the summit meeting, assuming that the Chinese would not act sooner in the interest of preserving a seamless conference. The appearance of ships while global leaders were still in Hangzhou was not expected.

The meeting had already been rocked by a chaotic arrival when the Chinese did not provide a rolling staircase for President Obama to disembark from the main door of Air Force One on his arrival in Hangzhou on Saturday.

Mr. Obama got off the plane by a staircase carried on board Air Force One and released from the belly of the aircraft.

Scarborough Shoal has been the center of attention as one of the most desirable places in the South China Sea for the Chinese to convert into an artificial island. It is close to military bases in the Philippines where the United States military has access.

In the last several years, the Chinese have built a group of artificial islands with military capabilities in the Spratly Archipelago, not far from Scarborough Shoal.