VIENTIANE (Reuters) - The Philippines expressed “grave concern” on Sunday and demanded an explanation from China’s ambassador over what it said was an increasing number of Chinese boats near the disputed Scarborough Shoal in the South China Sea.

A fisherman repairs his boat overlooking fishing boats that fish in the disputed Scarborough Shoal in the South China Sea, at Masinloc, Zambales, in the Philippines April 22, 2015. REUTERS/Erik De Castro

A Philippines air force plane flew over the rocky outcrop on Saturday and spotted more boats than usual in a flotilla China has maintained since seizing the shoal after a tense standoff in 2012, Defense Secretary Delfin Lorenzana said.

“There were four Chinese coastguard ships and six other vessels, including blue-coloured barges, around Scarborough Shoal,” he said in a text message sent to reporters.

“The presence of many ships other than coastguard in the area is a cause of grave concern.”

China’s embassy in Manila could not be reached for comment.

Although the shoal is merely a few rocks poking above the sea, it is important to the Philippines because of its tranquil waters and rich stocks of fish. Manila says China’s blockade of the shoal is a violation of international law.

The dispute has become more significant since the Permanent Court of Arbitration ruled on July 12 that no one country had sovereign rights over activity in the Scarborough Shoal, a traditional fishing ground for Chinese, Filipino and Vietnamese.

China has refused to recognize the ruling and the latest comment from the Philippines could cause a stir ahead of a regional summit in Laos starting on Tuesday, where leaders of Southeast Asian states as well as China, Japan and United States will meet.

Philippine President Rodrigo Duterte wants China to abide by the ruling but has pledged not to raise the issue during the summit. He wants to smooth the way for bilateral negotiations and last month sent former President Fidel Ramos as his special envoy to meet Chinese representatives in Hong Kong.

Lorenzana said Beijing had earlier this year tried to send dredging barges to the shoal, but there was no sign of any reclamation activity so far.

China previously used barges to dredge sand for its seven man-made islands in the Spratlys.

“We don’t know yet if those barges are precursors of future dredging operations,” he said.

“If they try to construct anything in Scarborough it will have far-reaching adverse effect on the security situation.”

China maintains claims to almost the entire South China Sea, which is a vital waterway for global trade. Malaysia, the Philippines, Taiwan, Brunei and Vietnam have competing jurisdictional claims.

A U.S. State Department spokesman said on Sunday the United States was monitoring the situation and encouraged “all sides to exercise restraint and take practical steps to lower tensions.”

U.S. President Barack Obama pressed his Chinese counterpart, Xi Jinping, on Saturday on territorial disputes in the South China Sea, urging Beijing to uphold its legal obligations and stressing the United States’ commitments to its regional allies.

Tensions over the disputed waters between China and its neighbors were expected to hang over the G20 summit, which opened in the eastern Chinese city of Hangzhou on Sunday.