While Duterte has embraced China and had largely set aside a once-tense standoff over the resource-rich waterway, a series of confrontations have stoked domestic discontent.

Duterte heads to China on August 28 and is due to return to the Philippines on September 2, his spokesman, Salvador Panelo, told journalists on Tuesday.

Beijing claims most of the South China Sea, including waters close to Philippine shores, and has ignored a 2016 international tribunal ruling that declared their assertion as without basis.

Duterte had earlier this month mentioned the trip, where he said he intended to finally discuss the ruling with Xi.

"I have about two more years, plus months left (in office). It's about time that we start talking," said Duterte at the time.

He also said he would press Beijing to conclude long-running talks with neighbouring countries, that also have claims to the disputed sea, on rules for avoiding accidental clashes.

The visit marks a turnaround for Duterte, who had revived once-icy diplomatic ties with Beijing after being elected in 2016 when he set aside the maritime ruling in favour of wooing Chinese aid, trade and investment.

Duterte enjoys firm popular backing, but he has faced criticism at home over his stance that confronting China is futile and will only lead to an unwinnable war.

The issue has flared up since a Chinese fishing trawler hit and sank a Filipino boat in the South China Sea in June, sparking a string of small street protests and criticism from opposition politicians and former officials.

Defence Secretary Delfin Lorenzana also complained earlier this month about repeated unauthorised passage by Chinese warships through Philippine territorial waters between February and July.

Panelo, the spokesman, said on Tuesday that Duterte has ordered all foreign vessels passing through Philippine waters to seek Manila's advance approval.

This is "to avoid misunderstanding in the future," Panelo said, adding this was in response to the "repeated passing through without our being notified by some foreign vessels -- well, particularly Chinese warships".

"Either we get a compliance in a friendly manner or we enforce it in an unfriendly manner," Panelo added.