China and Vietnam, which are involved in a tense standoff in the South China Sea, are pleading their case to U.N. Secretary General Ban Ki-moon.
Beijing on Monday sent a paper to the U.N. chief, accusing Vietnam of "illegally and forcefully" disrupting Chinese oil drilling in waters claimed by both nations.
In the letter, China's Deputy U.N. Ambassador Wang Min said Vietnam was violating China's sovereignty and posing a "grave threat" to Chinese workers.
Vietnam later said it sent its own letter to Mr. Ban, demanding Beijing immediately withdraw the oil rig and all other ships, which it said violate Hanoi's sovereignty.
Hanoi also called on China to "create conditions" for talks on measures "to stabilize the situation and control the maritime issues between the two countries."