MANILA - Philippine President Rodrigo Duterte said on Monday his order to the military to reinforce areas in the South China Sea controlled by Manila was to maintain the geopolitical balance, and assured China no "offensive weapons" would be placed there.

• PH won't put weapons in S. China Sea islands

Duterte said the Philippines wanted peace and friendship with China but his country needed to bolster what territory it had in the Spratly archipelago because "everybody's grabbing" islands and reefs in the disputed waterway.

The maverick former mayor set off alarm bells on Thursday, including in Beijing, when he said he had ordered troops to occupy uninhabited islets and shoals that the Philippines claims in the Spratly Islands.

Philippine officials later said plans were to upgrade existing facilities and not occupy new territories.

"I'd like to address myself to the Chinese government... I ordered the occupation of the 10 or nine islands that are just near our shores because there's a heightening of geopolitical issues and eventually maybe a violent low-intensity war over here," he told a news conference on Monday.

He said the Philippines would not engage in any military buildup, but indicated the United States would.

China claims most of the South China Sea through which about $5 trillion in ship-borne trade passes annually. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.

Duterte has turned Philippine foreign policy upside down by making overtures towards China and fiercely rebuking traditional ally the United States. Duterte wants China to be a major investor in the Philippines.

Duterte puts the blame for current maritime tensions squarely on Washington, for not intervening to stop China building and arming artificial islands in the Philippines' exclusive economic zone.

"If they fight each other, we will be hit. Everybody knows, the United States will be stockpiling their weapons there. And, they said they will not," he said, without elaborating.

"I do not want to get involved in a war between nations. I have extended my hand and friendship to the U.S. government."

He seldom criticizes China for its actions in the South China Sea and stressed on Monday that any future Philippine activities in the Spratlys would not be hostile.

"For the information of China, we will not place there any offensive weapons, not even one gun," he said.

China's Foreign Ministry spokeswoman Hua Chunying told a regular briefing there was "close and effective communication" between Beijing and Manila.

"We hope the Philippines can move forward with China and continue to appropriately handle disputes and create a good atmosphere," she said.