HANGZHOU, China — An unusually large number of Chinese vessels have been positioned close to a disputed reef in the South China Sea near the coast of the Philippines for the last week, despite warnings by the United States that China should stay away.

The Philippine Defense Department has photographs of four Chinese Coast Guard ships, and six other vessels, less than a mile from Scarborough Shoal, which is claimed by the Philippines and China, the defense minister, Delfin Lorenzana, said in an interview.

The presence of the Chinese ships during the Group of 20 summit meeting in Hangzhou, China, where President Obama and President Xi Jinping met on Saturday, seemed particularly provocative.

President Obama specifically warned Mr. Xi at a meeting in Washington in March not to start building an island at Scarborough Shoal. White House officials said that Mr. Obama planned to deliver the same message to Mr. Xi at their meeting in Hangzhou.

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.