China is demanding that the Philippines release nine Chinese fishermen who were arrested in a part of the South China Sea claimed by both countries.
The fishermen, who were detained in May near the Half Moon Shoal, were fined this week $100,000 each for poaching an endangered species of sea turtle.
China's foreign ministry spokeswoman Hua Chunying on Tuesday urged Manila to "unconditionally release" the boat and the fishermen. Hua said the men and the boat were "illegally seized" by the Philippines, which she said "violated China's sovereignty and rights of administration."
China claims the Half Moon Shoal, along with nearly the rest of the South China Sea.
Earlier this year, the Philippines filed a challenge with a United Nations tribunal over China's maritime claims. Beijing says it does not recognize international arbitration of the dispute and has refused to defend itself or otherwise take part in the proceedings.
Vietnam, the Philippines, Brunei and Malaysia all have overlapping territorial disputes with China in the South China Sea.