Saying he wanted to keep the "geopolitical balance" in the South China Sea, President Rodrigo Duterte yesterday ordered the Philippine military to fortify the islands and reefs it has occupied in the disputed waterway.

"We are just there to claim the islands for us because those are really ours... I have ordered the (military) to build structures there to signify that those are ours," Mr Duterte told reporters before he boarded a plane to Saudi Arabia for a week- long swing to the Middle East.

But he assured China that no "offensive weapons" would be placed on Philippine-occupied sites, "not even one gun".

Mr Duterte set off alarm bells last Thursday, particularly in Beijing, when he first disclosed that he wanted the military to fortify nine islands and reefs the Philippines is claiming in the Spratlys archipelago in the southern half of the South China Sea.

He also announced that he was planning a trip to to raise the Philippine flag on the largest island in the Spratlys that the Philippines has occupied.

China's Foreign Ministry expressed concern about the plans.

"I ordered the occupation of the... islands that are just near our shores because there's a heightening of the geopolitical issues and eventually maybe a violent low-intensity war over here.

"China can relax. We are friends. We will not go to war with you. We're just trying to maintain the balance of the geopolitical situation there," Mr Duterte said yesterday.

The Philippine leader added that he did not intend to engage in a military build-up, but "everybody's grabbing every island, every islet" and that a more direct conflict could arise between China and the United States.

"If they start fighting, we'd be caught in the middle," he said.

China claims most of the South China Sea. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims in the waterway.