MANILA—The Philippines said new surveillance images showed that Beijing is stepping up its presence in disputed South China Sea waters, raising the specter of a diplomatic clash as President Rodrigo Duterte attends his first regional summits.

Mr. Duterte’s government Monday summoned the Chinese ambassador to explain why Beijing had sent a flotilla of 10 ships, including dredgers and barges, to a shoal that an international tribunal ruled belonged to Manila. Defense Secretary Delfin Lorenzana said a Philippine military pilot had photographed the flotilla on Scarborough Shoal on Saturday. He said there was no sign of dredging work but the ships’ presence implied intent.

“If they try to construct anything in Scarborough, it will have [a] far-reaching adverse effect on the security situation,” Mr. Lorenzana said.

A Chinese Foreign Ministry spokeswoman said China “has maintained a Coast Guard patrol and there are some fishing boats undertaking fishing operations in the area. The situation hasn’t changed. We hope it won’t be hyped up, and that both sides can work together to build trust.”

During his two months in office, Mr. Duterte has generally shunned international diplomatic norms. He has berated the United Nations secretary-general, international human- rights agencies and the U.S. ambassador to Manila, accusing them of meddling in the country’s internal affairs by criticizing his bloody war on drugs and crime.

Yet Mr. Duterte has so far been more circumspect on China, as he seeks to repair relations that have reached new lows recently over the two countries’ territorial rivalry.

The revelation over the flotilla at Scarborough Shoal will test Mr. Duterte’s approach when he is scheduled this week to meet with Chinese officials and other leaders at the Association of Southeast Asian Nations Summit and the East Asia Summit in Laos.

“Why is China treating us this way?” said Mr. Duterte, according to Agriculture Secretary Manny Piñol, who attended a cabinet security briefing at which Mr. Duterte was shown the surveillance photos. In a Facebook post on Sunday, Mr. Piñol said Mr. Duterte was upset to learn that China seemed determined to provoke the Philippines, despite his conciliatory efforts.

Diplomacy is a new remit for Mr. Duterte. He spent most of his career as a mayor in the southern Philippine city of Davao, where he earned a reputation for taking a no-nonsense approach. His experience there is a world away from the tightly choreographed arena of foreign relations.

Beijing has rejected the tribunal’s ruling but made overtures to Mr. Duterte, inviting him to reset bilateral relations and open negotiations. He reciprocated in August by dispatching former President Fidel Ramos to Hong Kong to meet senior Chinese officials to prepare the ground for formal talks later this year. While making clear that he wouldn’t set aside The Hague ruling when joining China at the negotiating table, Mr. Duterte said he was eager to mend ties and enlist Beijing as a much-needed development partner. Last week he promised not to raise the sensitive matter of the arbitration case in Laos so as not to antagonize the Chinese.

The Department of Foreign Affairs in Manila didn’t respond to requests for comment.

China has dismayed its neighbors over the last two years by reclaiming land and building artificial islets in seven other locations in the disputed South China Sea, which is also claimed in part by Brunei, Malaysia, Taiwan and Vietnam. Chinese officials have consistently said that Beijing is within its rights to build within what it regards as Chinese sovereign territory.

Earlier this year, Beijing looked set to start similar construction work at Scarborough Shoal—a semi-submerged reef—until Mr. Obama warned in March that the U.S., a close Philippine ally, wouldn’t tolerate such a move.

Scarborough Shoal is located roughly 140 miles west of the main Philippine island of Luzon, making any future Chinese base there a potential threat to the security of the Philippines and the thousands of U.S. personnel who will soon deploy there under the terms of a recent defense agreement.

The shoal was an important source of livelihood for hundreds of Filipino fishermen until Chinese vessels took control of the area following a monthslong standoff with the Philippine Navy in 2012. They have been blocked from entering since then.

The South China Sea issue comes as Mr. Duterte is grappling with the aftermath of a terrorist bombing in Davao, which claimed 14 lives last weekend. On Monday, police said security forces were searching for four suspects.

Mr. Duterte on Monday bristled over a question on how he would react if Mr. Obama were to raise the issue of extrajudicial killings under the Philippine leader’s crackdown.

“The Philippines is not a vassal state—we have long since ceased to be a colony of the United States,” he said. He then spoke in Tagalog, referring to Mr. Obama with the slur.

Following the comments, Mr. Obama cast doubt on his meeting with Mr. Duterte, which had been scheduled for Tuesday, questioning whether it could be productive. His office later announced it was canceled. The U.S. president said he directed his aides to talk with their Philippine counterparts after learning of Mr. Duterte’s comments that Mr. Obama has no standing to confront him about human rights.

—Chuin-Wei Yap in Beijing contributed to this article.

Source: https://www.wsj.com/articles/sea-dispute-heats-up-before-asia-summit-1473073718