MANILA—President Rodrigo Duterte said on Tuesday the Philippines would not waver in defense of its interests in the South China Sea, even though he had barred his ministers from talking about the situation there in public. China’s maritime conduct has been a constant problem for Duterte but he has refrained from criticizing Beijing and instead praised its leadership, hoping to secure investment. But after weeks of rebukes of China by his ministers over the presence of hundreds of fishing vessels in the Philippines’ Exclusive Economic Zone (EEZ), he told his ministers on Monday they must refrain from discussing the matter. “If we talk, we talk but just among us,” he said in a televised address. On Tuesday, he said his order should not be construed as weakness and that maritime patrols must continue. “Our agencies have been directed to do what they must and should to protect and defend our …