Philippine President Rodrigo Duterte on Thursday ordered troops to occupy uninhabited islands and shoals it claims in the disputed South China Sea, asserting Philippine sovereignty in an apparent change of tack likely to anger China.

The firebrand leader, who on the campaign trail joked that he would jet ski to a Chinese man-made island in the South China Sea to reinforce Manila's claim, also said he may visit a Philippine-controlled island to raise the national flag.

"The unoccupied, which are ours, let's live on it," Duterte told reporters during a visit to a military base in Palawan, near the disputed waters.

"It looks like everyone is making a grab for the islands there. So we better live on those that are still unoccupied. What's ours now, we claim it and make a strong point from there."

Duterte's plan is unlikely to sit well with China, which lays claim to almost all the South China Sea, especially as it comes amid a fast-warming relationship in recent months.

China, Malaysia, the Philippines, Taiwan, Brunei and Vietnam contest all or parts of the South China Sea. This has led to confrontations between China and some of its neighbours over the strategic trade route.

Duterte's comments came just ahead of a first summit meeting between U.S. President Donald Trump and Chinese President Xi Jinping in Florida on Thursday and Friday. China's pursuit of territory in the South China Sea will be among the pressing security issues on the agenda.

The U.S. State Department declined comment on Duterte's remarks, but has in the past urged rival South China Sea claimants to lower tensions and resolve differences in accordance with international law.

Duterte announced his "separation" from the United States in October, declaring he had realigned with China as the two agreed to resolve their South China Sea dispute through talks.

His efforts to engage China, months after a tribunal in the Hague ruled Beijing did not have historic rights to the South China Sea, in themselves marked a reversal in foreign policy.

The Philippines occupies nine "features", or islands and reefs, in the South China Sea, including a World War II-vintage transport ship which ran aground on Second Thomas Shoal in the late 1990s.

Asia expert Bonnie Glaser at Washington's Centre for Strategic and International Studies think tank, said Duterte's remarks looked like "bluster" comparable to his jet-ski remarks. "Vintage Duterte. All bark, no bite," she wrote on Twitter.

"Duterte seems to be under greater domestic pressure and criticism on maritime issues," she added to Reuters. "I’m inclined to believe that his latest comment is driven ... by domestic politics."

Duterte said he might visit the island of Thitu, the largest of the Philippine-controlled islands in the Spratly archipelago, and build a barracks for servicemen operating in the area.

The Philippines marks its independence day on June 12.

Thitu is close to Subi Reef, one of seven man-made islands in the Spratlys that China is accused of developing as military outposts.

Last month, Defence Minister Delfin Lorenzana said the Philippines would strengthen its facilities in the Spratlys by building a new port and paving an existing rough airstrip.

Duterte said last month it was pointless trying to challenge China's fortification of its man-made islands and ridiculed the media for taking his jet-ski comments seriously.

"We cannot stop them because they are building it with their mind fixed that they own the place. China will go to war," he said.