MANILA, Philippines (AP) — An international tribunal has asked China to defend its territorial claims in the South China Sea by submitting written arguments within six months despite its refusal to meet a Philippine legal challenge to those sweeping claims.

China has refused to join the arbitration process initiated last year by the Philippine government. Philippine officials on Wednesday reiterated a call to China to join the arbitration as a peaceful and durable solution to resolve the long-raging territorial disputes.

The Hague-based tribunal issued a statement Tuesday giving China until Dec. 15 to submit arguments and evidence against the Philippine complaint, which questions the validity of China’s so-called vast “nine-dash” claim, which refers to a rough Chinese demarcation of its territorial claims that cover virtually the entire strategic waters.