Philippine President Rodrigo Duterte has banned his cabinet from speaking out in public on the South China Sea dispute, after key ministers engaged in a war of words with Beijing.

Tensions between Manila and Beijing over the waterway -- which China claims almost entirely -- flared in March after hundreds of Chinese boats were spotted inside the Philippines' Exclusive Economic Zone.

While Duterte has been reluctant to confront China over the issue, his foreign and defence secretaries have repeatedly criticised Beijing for its refusal to withdraw the ships from the disputed waters.

Earlier this month, Foreign Secretary Teodoro Locsin tweeted an expletive-tagged demand for the Chinese vessels to leave the area.

His online swearing prompted a rebuke from Beijing and Locsin later apologised to his Chinese counterpart.

Philippine President Rodrigo Duterte has banned his cabinet from speaking out in public on the South China Sea dispute
Philippine President Rodrigo Duterte has banned his cabinet from speaking out in public on the South China Sea dispute Philippines' Presidential Photographers Division (PPD) / Robinson NINAL

"This is my order now to the cabinet... to refrain (from) discussing this West Philippine Sea (issue) with... anybody," Duterte said in a recorded speech late Monday, using the local name for the sea.

"If we have to talk, we talk only among us," Duterte told several cabinet members, including Defence Secretary Delfin Lorenzana who previously described the presence of Chinese boats as an "incursion".

Presidential spokesman Harry Roque was allowed to address the issue in public, Duterte added.

China has ignored a 2016 international tribunal decision that declared its historical claim over most of the South China Sea to be without basis.

Duterte has set aside the ruling in exchange for promises of trade and investment from China that critics say have largely not materialised.