The U.S. will remain the most important partner for the Philippines even as Rodrigo Duterte improves ties with China, his foreign secretary said ahead of the president's trip to Beijing.

"We have a special relationship with the U.S.," Perfecto Yasay, the country's top diplomat, said in an interview Thursday in his office in Manila. "They are our only military ally. You could not put the United States, in that sense, in equal footing with China."

The U.S. has been the Philippines' closest ally since independence in 1946, and the nations are linked by formal defense treaties. But those ties have been strained since Duterte took office three months ago, with the tough-talking leader frequently calling the relationship into question.