MANILA, June 1 -- The Philippines and Vietnam on Monday declared peace, stability, and freedom of navigation in the South China Sea to be "non-negotiable" as the two claimant states elevated bilateral ties and renewed a key defense cooperation agreement.

President Ferdinand R. Marcos Jr., following bilateral talks with visiting Vietnamese President To Lam, said both countries, which are among the claimant states in the disputed waters, agreed that preserving stability in the South China Sea is "non-negotiable."

"As fellow claimant states, we reaffirm that maintaining peace, stability, and the freedom of navigation and overflight in the South China Sea remains non-negotiable," Marcos said.

The statement came as Manila and Hanoi renewed...